2004.5+ LLY Duramax Fuel Economy Poll

[More Power]

Fuel economy is one of the top 2-3 reasons why most people buy a diesel pickup.

What we want to do here is help determine what people are seeing for non-towing fuel economy in actual on-road use.

Let us know what your LLY Duramax powered 2500HD/3500 truck delivers.... This poll is for owners of the 2004.5 or newer LLY. We are assuming summer blend fuel.

You will need to vote before viewing the poll data. If you don't own an LLY, but would like to see the poll data, you'll need to first vote in the poll using the "Don't Own an LLY" selection for both the local driving and highway driving sections.

Round your fuel economy averages to the nearest whole number. For example a 16.4-mpg would be 16, a 16.8-mpg would be 17.

MP

[ 01-03-2005, 02:33 PM: Message edited by: More Power ]

[Kennedy]

Here again, I voted a bracket down. I have seen 18.8 and 18.9 hwy, but couldn't justify the "push" to the 19-20 bracket. Not bad for only about 3k on the clock, and no bed cover yet!

[DURA-MAX3]

In 8800 miles mine is averaging 12 mpg. So far I am not impressed but it runs decent. I am going to put a chip on it and have fun. I figure if it gets poor mileage might as well make run like the wind.

[J-M]

I voted but have a 4500. Alot heavier of a truck.

[carco]

I wonder just how much a K2500 DA can pull?? I pulled 10000# yesterday 100 miles several hills WOW it pulls!! 8000# 38'gooseneck enclosed w/ 4 ATV's, 1 dirt bike, gear and 4 people. Southern Ohio, N Ky. hills, both sides of the Ohio river. I thought mileage would be 12-13 but was only 10.7 mpg for 104 mile round trip @ average 43 mph. A little disapointing for 17500gw, my C6500 w/ Cat 7.2L @ 10.88 mpg over 804 mile trip @ 68-72 mph to GRapids Mi. @ 20000#.

[Cliffee]

I'm averaging 16.5 with mostly short city trips. 1800mi on OD. 2005 LLY

[MPH]

I too have been a little dissapointed in the lly's mileage. driving home from the dealer it averaged 18.4, which I was impressed with, but it has never been that high since. Pulling a little two place snowmobile trailer, i was only seeing about 11 mpg. I put 305's on it right when i got it home. could this have affected it that much? It didn't seem to drop the LB7 that far.

[HalsMax]

I just took a 400 mile trip. Averaged 19 cruising between 70-75. Programmed the Hypertech PP to stage 2 and she got 17 on the same trip, but was a lot more fun passing on two lanes. I have under 5K miles and am impressed with the mileage and pulling power. Gonna leave her on stage 2!! :D

[ 01-02-2005, 07:12 PM: Message edited by: HalsMax ]

[Kennedy]

Having never owned a truck with an avg economy computer, (DIC) it has proven enlightening. While accuracy is sometimes not the greatest, and is really thrown off with many tuners/modules, it DOES give pretty good indication of what happens when you blast past csomeone or let the truck idle, or even drive off cold and the trans is in cold shift mode etc..

In order to get decent mpg you need to excercise restraint when passing, and minimize cold engine and idle operating time.

[dcalex]

You gotta be careful in looking at the percentage results on the poll....the question directed to the people who don't have a Dmax (which is the majority responding) is included in the percentages and it skews the results.

To date the percentages without the skewing question is this:

less than 15 mpg........5....11%
15 - 16.................14...30%
17 - 18.................17...37%
19 - 20.................10...22%

Thats 59% getting better than 17 mpg....not bad at all for that powerful an engine. smile.gif

[Lee_klm]

I selected 19+ on Highway, but would need to qualify my answer as under 60mph driving. 70mph will keep me at 17, maybe 18 with the wind...

[belcll]

Originally posted by MPH:
I too have been a little dissapointed in the lly's mileage. driving home from the dealer it averaged 18.4, which I was impressed with, but it has never been that high since. Pulling a little two place snowmobile trailer, i was only seeing about 11 mpg. I put 305's on it right when i got it home. could this have affected it that much? It didn't seem to drop the LB7 that far.

[belcll]

oops, did a false post. I added 285/75 tires and notice my mileage fall almost immediatly by about 1-2 mpg.



Originally posted by MPH:
I too have been a little dissapointed in the lly's mileage. driving home from the dealer it averaged 18.4, which I was impressed with, but it has never been that high since. Pulling a little two place snowmobile trailer, i was only seeing about 11 mpg. I put 305's on it right when i got it home. could this have affected it that much? It didn't seem to drop the LB7 that far.

[Freeman777]

I participated in this survey (15 - 16 mpg city and 17 - 18 mpg hwy) a day or so after the poll was posted. I've been back several times since and noticed that the majority of the LLY owners fall into this category. I've been in the high 18's on several occasions, but didn't feel justified in voting in the 18 - 19 mpg class. Yesterday, on hwy #95 (just North of Las Vegas, NV) I ran a test under ideal conditions of road, wind, terrain, etc. and marked 20 miles at 70mph on the cruise control. The result was: 23.2 mpg according to my digital information center. My rig is a 2004.5 GMC C3500 CC, SLT, LB, 4WD D/A (LLY). After 6 mo. of use I now have 15K and the only thing I've been doing differently is using a fuel additive: Power Service -8 oz. per 25 gal. (states on the container: "absolutely no alcohol") Although this truck is running GREAT! I wasn't happy pulling a 36', 14+K, 5er over Baker grade (I-15 bet. Stateline, NV and Baker, CA) in 2nd gear, at 35mph at 3,500 rpm and both temp. gauges too close to hot for my comfort. Trips like that to San Diego resulted in 10mpg. Since my last trip, I've been anxious about the new Bank's set-up (in R&D for quite some time) for my LLY and got the GOOD NEWS call today: A six stage system (air filter, six-gun controller, exhaust system, boost and pyrometer gauges) that will selectively and progressively boost my engine/trans 128 HP and 345 fp torque. Stage #1 is the stock setting and every stage thereafter = a 20% boost. By going with the unique design of the Banks set-up, I think my engine/trans plus my 100,000 mi. after-market warranty will be better protected.

[eagle in flight]

My LLY has been getting about 18 mpg all by itself. When I tow a fifth wheel (weight around 14,000 pounds) and travel at 60 mph, I get around 11.5 mpg. Mileage is at 15,000.

[fishhead1076]

At first very disapointed with the lly and fuel milage, my average was around 12 no matter what towing or mt. After 12K and first fuel filter change my milage changed drastically, went from avg. of 12 to 17 in the city and 18+ on the highway. Didnt really buy into the break in better fuel milage bit, but I must say I was suprised.

My 2 cents...

[tanker]

Well just picked mine up last Monday night, did some round town back and forth to work running, then stretched its legs down to Virginia this weekend and the computer says 16.6mpg, not so bad running 70mph, Opps :rolleyes: not supposed to go over 55mph in the first 500 miles, but ran well, didn't do any major acceleration's just cruised along with traffic. Should improve as it breaks in. First 100+ miles in stop and go driving, some high idle warm-ups. Will wait and see. Can't wait to hitch up and go. ;)

[jogger]

Did a short 150 mile round trip consisting of mostly freeway and rural driving and averaged 21MPG for that trip. In the city I average about 18.

[guerrero]

First 430 miles coming back from the dealer in Kellogg Idaho with outside temps hovering in the high 20's and several inches of snow on the ground through one of the mountain passes requiring prudent use of 4WD on I-90, averaged 20.5 mpg. Very happy with these results and expect additional improvements when I get the truck back to PH Hi and the warmer climates.
2005 PDN extended cab

[mjammer]

I am seeing 17.1 for mixed driving on my new truck, 2500HD D/A 4WD CC LB (just picked it up Friday) in the first 200 miles. That is better than I ever saw on my old gas 2500 extended.

[Mile Hi Guy]

Well I have roughly 17,000 miles on her now I got 16.0 twice (I do check every tank manually) but usually average low 15's empty driving mostly a 80% Hwy commute. Traded my brother one week (10 days actually) and got 17.2 doing the same routine in a 2001 AWD Denali :mad:

Truck runs ok but not nearly as strong as by Brother-in-laws 05 CCSB Ford 6.0 :confused:

Had the truck in twice and they say everyting is ok in the motor department. The only thing they cannot seem to fix is the Drivers seat will not keep it's memory and a whistle on the passenger side door. Would that qualify for a lemon? ;)

[Shire Ben]

I have a 2004.5 LLY which I purchased in Mar 04 with 24K and I consistently get 19 mpg hwy. No modifications or upgrades. LOF every 3K and FF every 15K. Had similar experience with 2001 LLY which 85K, same maintenance, no injector problems..Thanx

[pepper]

I have gotten 22 mpg on a tank full but I had to keep my foot out off the accelerator, 62 mph or slower and no repeat no hard acceleration, that is no fun so usually hit a few times per tank and get 18-19 - towing the boat i get 13-14 it is a wide flats boat with center console which is more of a wind trap than a weight issue, heck i only got 12-13 highway with my old gas van, wish they had a van with this eng/tran setup. I'd have one tomorrow, yes w/4wd!

[Dewey Faircloth]

Get 18 to 19 driving empty 13.2 pulling travel trailer staying about 65.
Leaving Saturday going to Camp Lejuene pulling trailer and will see if it does better now has over 12,000 miles on it,fresh filters and gravy.

[Jerry D]

Today's 50 mile journey of back roads and HW had me average 22 MPG according the DIC. I'm wondering if when I filled up a couple of days ago I got my first tank of Non winter fuel because this is better than I've been getting recently.

[Curt B.]

Just wondered how fast most of you drive, that are seding 18+ mpg? I drive 75-80 on interstate most trips, and I have yet to get over 17. Today with 30 mph tailwind, DIC was showing 20-21. I never get that on a normal trip.

[KCDuramaxTurbo]

300 miles so far 05 lly, first tank, home from dealer 20.0, now 18.2. Really easy on the accelerator. Have a predator coming.

[Noal]

'05, 4x4 2500 HD Crew Cab. 3000 miles. Getting average or 16mgp town or highway. Had '02, same truck except 8.1 L gas. 60 % better milage. See the smile on my face.

[Shoe]

First post on the forum...

I'm sure hoping my the mileage on my 05 will improve as it breaks in. I'm milking it in order to achieve anything over 17. (still under a 1000 miles) The 03 was getting a steady 19 from the dealer and ended up with 21 at 65K. Sure seems the turbo kicks in at a lower RPM on the new one and it runs a little warmer than the 03 at higher speeds.

Great Forum!

[Wally1]

05 CC/4X4. About to fill up for my third tank.
So far 13.4 mpg all city driving. :(

[mjammer]

Wally, that is similar to what was seeing except a little lower as it was cold weather in February. Find an excuse to take a couple of log interstate trips to help break it it at more than just highway sppeds and see what it does on the interstate. My local driving mileage has seemed to pick up after some longer trips and the associated mileage. I am at 2500 miles now and see low 18's on interstate running ~68-69 MPH. My local mileage has picked up 1-2 mpg now.

[JD Diesel]

2000 on LLY 14 t0 16 Lot better than my 2500 sub with 454 vortec 9 to 11

[reedpilot]

I have a brand new 2005 2500HD with almost every option and I am getting 15.8 around town and 21.9 on the hwy. What are people using to increase fuel economy?

[AndyU]

I took delivery March 31. I have about 3500 miles on already. I get around 17 in the city and as much as 21 highway, unloaded. Average unloaded highway mileage at ~75mph is between 17 and 20 with wind conditions making a big diffence. With a tail wind I will break 20. With a head wind... 17. Same pattern pulling my gooseneck horse trailer. Anything from 8.5 to 11.5 depending upon wind conditions at 65 to 70 mph. The trailers pretty big and heavy though, 8' wide about 9' high x 34' long, fully enclosed aluminum and stainless steel. 2-3 horses, water and hay add another 2-3 thousand pounds.

I just bought a Predator and on Sunday I loaded the 40hp tow program. I pulled my horses on a little 160 mile trip and the DIY said I got 11.8 pulling into a head wind @65-70 mph. Not bad I thought.. about 2 mpg better than usual into a stiff headwind. Next I unloaded two horses and reset the DIY. I then pulled my empty trailer another 40 miles down back highways at 55-60 crossways to the wind. It said I got 16 mpg. AWESOME! I thought (about 4mpg better than usual!). Finally, I dropped the trailer, reset the DIY and ran another 40 miles with the just the truck at 75 (no wind by then) and the DIY said I averaged 25mpg... that's when I got skeptical... to good to be true. I drove to work and back today, another 20 miles (I usually get about 17 to work and back). I refilled the tank and it took 21 gallons. 260 total miles / 21 gallons = about 12.4 average mpg. That was obviously way off. It should have used about 18.7 gallons according to the DIY for around 13.9 average mpg. The DIY used to be accurate within about 1/2 mpg and usually showed less than my actual. Now it's way off the other way. Despite the impressive computer reports, my real fuel economy appears no better...

Does anyone know why the DIY would get so far off? Does the predator change the calculations?

I'm going to continue testing for another week. If the predator provides no improvement over stock economy, it's going back. If I can get an extra 1 mpg or better, then it's well worth it, especially since it won't over stress my truck. The simple 40 hp tow program is as hot as I care to get.

I really baby the truck when I'm driving, soft stops, soft starts, rpm never gets above 2500. I really drive to maximize my fuel economy and equipment life.

Has anyone else seen improved fuel economy with a programmer? I have a friend who gets 23 with his 05 stock cummins and 15 pulling a trailer like mine. I'm on a real quest to get similar performance.

[okeehandyman]

have 2 '05 2500HD 4x4, 1 c cab 6' box & 1 ex cab 8' box. both getting 17-18 intown & 21-22 on out of town trips around 60 mph. about 1.5 mpg less per 10 mph faster. the fact is you have to decide how fast vs how much fuel to use. no free lunch, remember.

[AndyU]

Well, after thorough testing, towing, empty, highway, city, etc, my mileage went down with the Predator. Running empty, I lost about 1.5 mpg in city driving and about the same loss highway. Towing was about the same, maybe even a bit better, but it's hard to tell because of the affects of wind direction, rate of travel, etc which makes it hard to get a good baseline. The Predator caused my DIC to indicate my mileage went way up, but hand calculating my fuel usage revealed that in fact it went down. Very disappointing.

After restoring the factory program I averaged 19.5 mpg on a 550 mile trip last weekend (empty at 70 mph w/ cruise and a tail wind). I got 22 mpg on the last 100 miles (empty, no wind, 60-65 mpg w/out cruise) after refueling. I think that's pretty good.

The truck definitely gets better mpg w/out cruise on, letting the truck slow up a bit ascending hills and speed up a bit going down. Also, if I keep it at 65 or less my mpg is much better. Also no hard acceleration.

I only have 5000 miles. I'm very happy with this truck. :D

[57CHEV]

I consistently got 16 with my 02 LB7 and 14 with my 04 LLY all in town driving. Exact same trucks except one was a Chevy & one was a GMC.

[bamabadboy]

19.8 MPG on a 800 mile trip. Unloaded, all stock, air on, driver only, bed uncovered, 65-70 MPH all the way, 271 miles on the odometer at start.

[Shorty]

Purchased 2002 HD 2500 DA crew cab in Jan 05 LOVE THAT TRUCK getting 18 average empty around Texas.Came to Colorado towing 28ft. Jayco got 12mpg towing trailer. purchased Predator installed 2nd highest economy tune getting 20 mpg average.went from Grand Junction to Ft. Collins about 600 miles average 21 mpg best I have done so far. Haven't tried towing yet. EGT got around 1200 going over Vail and Loveland passes had to ease off of cruise to maintain EGTs

[RodneyRam-jet]

I picked up my '05 K2500HD CC SB 6/13/05. Used 18.895 gallons to go first 270 miles. I will fill up tonight or tomorrow on second tank full. That puts me at 14 mpg on first tank and I can tell I am doing better on second tank. Taking my first trip towing in two weeks. Does anyone use fuel additives? Any recommendations? I heard it's good to use during break-in.

[G Slaton]

I have an 05 CC 2wd Duramax/Allison. This is my fourth Duramax. By far the 03 got the best fuel mileage and ran just about as good as the LLY. I hae driven several LLYs and all run better than mine, local diesel tech says it happens and no real reason why. Running at cruise control 70 It will get a best of 14.7 and a headwind will knock it down below 14. I dragrace and tow a 42ft enclosed gooseneck hauler(14,000#). Normal mileage is 8.9 towing 67 mph.. Headwind will take it below 8.0 I check all fuel mileage by filling to the neck and useing the trip odo. The truck indicator is usually within .1 to .2 mpg of the actual. This truck has 20,000 miles on it and the fuel mileage has not improved with age as I have towed with it from 500 miles on. The LLY has been a bit of a disappointment. I have owned Chevy trucks all my life and am not about to change but this particular truck is not the best one I have ever owned. My LB7 trucks would get 10 towing the same trailer and have been as high as 11.5 on no wind level ground.

Gary

[LJ]

Hey guys, this is my first post, my first diesel pickup, and so greetings to all.

My truck is a 2005 2500HD w/ Duramax bought 6/1. The first 1,000 miles, the DIC said I got 17.4 overall. When it turned over 1,000, I reset it. Since then, overall, it's been 19.1.

Good points on slowing down above. That Allison tranny won't let me coast. I can see that driving habit changes will help.

[6APPEAL]

Purchased a 2005 Chevy 2500 HD Crewcab Duramax/Allison 2wd on 6/18. Just about ready to fill up my 1st tank and have been getting between 16 and 17 mpg according to the DIC with mostly city driving. I'll manually check that upon filling up to see how close the DIC is. I've been taking it easy, staying under 2500 rpm. Taking a trip this weekend and should be above the 500 mile mark listed in the owners manual, so I'm curious to see what kind of milage I'll get with all highway driving around 70 mph. This is my 1st diesel, which I bought for the towing capabilty. But, I will say it's getting within 1.5 mpg of my old 4.8l C1500 extra cab for the same daily work/home driving. Should exceed the gas burner while towing and not be as hairy to drive.

[6APPEAL]

1st tank of fuel last night. Figured my mileage (361 miles on 19.61 gallons), 18.4 mpg. All I can say is WOW. The DIC was off about 1.5 mpg, but who cares. That is the same mileage I was getting with my 4.8 gas burner in the same driving conditions. I'm interested to see if it get the same on a road trip. The gas burner would get around 23 (70+, air on, unloaded). I know the diesel will blow it out towing.
John

[dieselmaybe]

Just bought a new Chevy 2005 2500HD LT Duramax/Allison yesteday. Had 3 miles on it when I left the dealer. I reset the DIC after driving home, about 25 miles, and checked it with the cruise control set at 60mph, it climbed to 18.5 after about 10 miles. How accurate is the DIC reading? Just curious, I intend to refill the tank and check it the old way this weekend. I test drove the Ford and Dodge and liked the Chevy best. This is a very nice truck. Also, I have noticed a lot of discussion about tire size. The dealer where I bought mine has 265/75s on his demo and said he thinks it lowers the power and fuel economy but shouldn't hurt anything or affect the warranty. I would really like to upgrade these 245s to 265s but don't want to lose much fuel economy, any opinions would be appreciated. (It doesn't seem like a 3.6% increase in diameter should have much effect)

[dieselmaybe]

Checked the unloaded mileage the old way on my new 2005 Chevy D/A. In town, 100 miles 13.8 mpg. 200 mile highway trip Saturday, driving fairly easy 55-65 mph it got exactly 19 mpg. The DIC was fairly accurate also. It was saying 13.5 in town and 19.5 on the highway. I also upgraded to Cooper Discoverer ATR 265-75 E rated tires. I couldn't stand those little 245 tires . Speedometer is off a little, at 60 mph my GPS says 61.2 mph. Before changing the tires the GPS read 59 when the speedo read 60. I'm liking this truck more every time I drive it.

[Bill W]

I have a new 05' extended cab regular bed. with 300 miles on the truck I am getting 14.5 on the city, 21 on cruise control at 60 mph per the display

[VANBC2500HDDA]

I just got a new 2005 Dually Crew SLT 4x4, and so far the trip computer is scaring me, average is 10MPG....

Does the mileage really fall off on the 1 ton trucks?

My last truck was a 2500HD

[cruzar]

I just received my 2005 2500 hd 6600 and took it for a road trip kept it at 65 - 70 and if the computer is correct it was steady at 18.9 and on the ranch with hills and draws in 4 wheel it was at 17.8 I am pleased at this time.

[bthompson]

I have had a 2004.5 LLY 2500HD Duramax for a little over a year. I have a little bigger tires than stock and have heard wonderful stories about gas mileage. As a matter of fact, I have three different buddies with Duramax's 03' or later that have the LB7 engine and average 16-17 around town and have gotten 20 on the highway. I looked at my buddies lifetime average the other day and his was 16.5!!! His is about a year old, but he has the LB7. The guy at the dealership told me the LLY should actually get better than the LB7. What is the deal? My lifetime average reads 12.4 mpg and I have never gotten better than 15 mpg on the highway. I also have an Edge/Attitude performance chip.
I have also read all of these posts about 16-17 mpg and even 20 mpg or better. Most of my miles are unloaded with bed covered, and I do not have a lead foot. Could there be that much difference in engines? Any advice, help, or answers?

[cruzar]

I have just put the second tank of fuel in my 05,
2500 HD long bed and the fuel mileage was slightly
lower than the first tank. On the same road that averaged around 18.9 I now am averaging 17.3. I kept the computer on, just like the first trip, and kept it at 2000 rpm/65 miles per hour. I do consider this a difference. I hope the fuel mileage does not keep going down instead of up. The first tank was filled by the dealer and I wonder if they put an additive to help the octane level of the fuel I also was told that there are to octane levels, 40/44 does anyone know if this is correct.

[Kelly T]

Had 02 1500HD 6.0 gas Crew Cab 4x4 shortbed. Bought 05 2500HD Duramax Crew Cab 4x4 shortbed. Kept fairly tight records on the 6.0 and have modified my spreadsheet to include the new gee-whiz factor of the 05 (automatically calculating data on the fly). Make sure you scroll down to where averages/min/max is calculated too.
First post, first diesel - all stock performance-wise, short bed with no cover - yet.

http://home.columbus.rr.com/vfrket/Diesel/Diesel.xls

My driving is mixed between around town & highway commute to Columbus (OH).

Average on the gas 02 was 14.14 MPG after 60K miles - again no bed cover. Average on the 05 Diesel is 18.73 - less that 3000 miles presently. The gee-whiz factor of auto-calulating mileage does keep me from pressing the accelerator as hard on the diesel as I did on the 6.0 gas.

Having fun so far. Looking to improve shift points - revs too much at 25, 35, and 45 mph, would like it to shift sooner - will likely go the route of Juice w/Attitude to improve mileage and shifting (to my liking). A little more power now and then never hurt. Typically hauling/trailering is minimal and usually just a motorcycle or ATV. E.G. I raced at Mid-Ohio this past week and hauled my racebike, spares, & associated gear.

Kelly

[Black95TD]

Mine is new - only 800 mi. now. I filled up on way home from dealer and then refilled before a 400 mi trip. 1st 1/4 tank (same station, same spot and tried to fill to the neck) 17.4 mpg rural driving. 30 - 50 mph, some stops, up and down the hills.

Next time, again refilled at exact same spot. 385 miles from Connecticut to upstate NY and back. 2/3 highway at 60 - 65 mph and 1/3 local roads around the lake. 21.1 mpg. I was taking it pretty easy so this may be the best I ever get.

Next tank is looking good too - at this point over 200 miles at 3/4 tank. I haven't reset the fuel indicator but it seems like its pretty darn close.

All in all, exceptional fuel mileage! I considered a gasser but everyone I talked to said they got 10.5 - 12 mpg. The trick will be to take it easy. I'll bet those injectors really pour the oil to it when you jump on it.

[Steve Rapp]

I just drove my 04.5 D/A from San Diego to Spfld Mass. and back and got almost 16 mpg pulling a loaded car trailer. I did a 500 mile round trip trailer less while in Mass. to Pa. and got 20 mpg and the AC was always on. I did 65 mph most of the time.

[fourtenposi]

I have a 2005 2500hd d/a with a high bed cap and all I can say is WOW. The power combined with the fuel ecomony makes this an awesome truck. I asked my buddy how the truck pulled my 26 ft enclosed trailer with the car and he replied "what trailer". Man he was not exagerating. Tons of power. I found at 60 mph on a level grade my truck dic says 23 mpg.It droppes to about 19 at 75. These trucks have alot smaller power band that a gas vehicle so fuel economy will suffer the faster you go. If you want the best fuel economy you need to keep the rpms at peak torque with I believe is 1800 rpms. The engine is most efficent there, that is why it has the most torque there.

[cruzar]

I have noticed a difference in the fuel since I filled up the last tank. This time I filled up at a station that touted premium diesel and my mileage per gallon increased again. This time running at just over 65 to 70 mph I am getting 18.3. This with having to run some hills but not pulling a trailer.I do feel that the quality of fuel and the type of driving has alot to do with the economy. I have enjoyed the vehicle so far, it has been a while since I have had a new vehicle. I now have just a little over 1500 miles on the odometer, the miles are cranking up a little to fast for me, don't want to use it up all at one time.The fuel here is around 2.35 to 2.37 per gallon, I am in Fort Worth Tx.

[daleslad]

This is my first deisel and first post. I don't know anything about deisels so I've read all of these posts with interest.
I bought mine because I wanted better gas mileage and to pull a 5th wheel. I bugged a lot of campers who towed with the duramax about how they liked theirs. One camper even let me drive his around the campground. I was impressed with how quiet and smooth it was .
However I'm not a happy camper, My 2005 has 2500 miles on it and I'm not getting very good mileage out of it.
My first problem was smoke coming out of the exhaust in clouds while driving solo on the freeway and also a rattling noise from under the hood.
The dealer flushed it out and tweaked the software and since then the engine has sounded much louder.
My latest problem has been a tendency to stall when going up a slight incline solo while trying to keep the speed at around 5 mpg. It won't actually stall but will recover and jerk forward which keeps me busy feathering the accellerator pedal.
The same problem manifests itself when I'm trying to back my 5th wheel up my driveway. It can get pretty scary as I don't have much room and it's like threading a needle and I don't like it jerking ahead like it does.
The dealer's answer is to tell me to put the 4 wheel drive into the Low, Low gear when backing up with the trailer.
I had a 2001 Sierra HD 2500 vortec 8100 before this truck and I never had any problems backing up my driveway. My trailer only weighs about 8,000 lbs. max.
Also when I'm pulling the trailer hard up a hill the engine sounds like a threshing machine. If it was a gas engine I'd say the tappets were rattling but I'm not sure just what a deisel is supposed to sound like.
If anyone has any advice or suggestions I'd surely like to hear them.
Thanks everyone it's nice to be able to share experiences with fellow deisel truckers.

[dieselfish]

I'm getting 14.2 avg which is about 1/2 surface streets and 1/2 hwy for my 12 mile work commute.
I now have 3K on a 2005 LLY. The only "real" hwy driving I have been pulling a 9000# 28' 5th wheel and getting about 10.5mpg between Phoenix and Payson AZ. About five 6% grades on the road at over 100deg ambient temp. Tried not to answer the highway because of not running unloaded but it did not like that. I choose 15-16 hwy only because I have yet to see anything over that.

[Kelly T]

Spreadsheet has been updated. Still in the high 18s for MPG.

No new toys on it - still stock. Have just (today) change oil & filter for second time and the transmission filter.

I continue to be impressed with the quality of the post in this forum. I went ahead and joined the members area. I am learning a lot about diesels. Thank you!

[ 08-28-2005, 03:21 PM: Message edited by: Kelly T ]

[cruzar]

You stated that this is the second oil, filter change. Did you change it yourself? if so what filters did you use, I think that someone stated that the oem filters were having problems if so please inform if not direct me to what filters you are using.

[More Power]

daleslad,

Your engine/fuel system has a problem. A competent and motivated dealer service department is what you need most.

I would suggest you begin a new topic thread with your comments about engine running issues. It'll get more eyes....

Jim

[Bugs]

2005 GMC Sierra Duramax/Allison. I do almost all city driving and I average about 14.5.

[Kelly T]

Originally posted by cruzar:
You stated that this is the second oil, filter change. Did you change it yourself? if so what filters did you use, I think that someone stated that the oem filters were having problems if so please inform if not direct me to what filters you are using. Yes changed it myself both times. Using factory recommended AC filter. Have not seen any posts about issues with it. Only posts I have seen related to the odler style FUEL filter. My D/A CC is 2005.

[cruzar]

Thanks, on the looking into the AC oil filter, how about the fuel filter? I have about 3000 miles on the vehicle now and I am monitoring the computer on the oil filter/ fuel filter change.Just looking for some feed back on the filters being used by some on this board.

[rob@rone.ca]

Has anyone computed the averages for the different models and options ie 2500 or 3500, SRW or DRW, allison or manual, etc???

I'd be real interested to know other peoples comparison with the 6.5 TD's

I had a heavy-duty K2500 6.5 TD single cab standard, and used to get 19-20 mpg empty (converted to us gallons)

With the LLY as below I'm getting 16.5-17 empty in similar driving after 4000 miles on it now.

[mjammer]

I just got back from a 300 mile round trip that was interstate and some 2 lane highway and got a new best of 21 MPG (calculated). Previous best was ~18.5 MPG. Around town and mixed driving (including some boat towing) I am averaging about 15.5 MPG. I am at almost 10K miles now. Mileage is slowly climbing over time as it continues to break in.

[LJ]

Guys, you have to hear this.

I knew when it got broken in better it would get better fuel economy, but this past Sunday I really was amazed.

My mother lives in West Union, WV. I had been visiting her, and was coming home. Two miles from her home, I reset my DIC and thought I'd take my time coming home. It's mostly all 4-lane, but thought I'd put it on cruise @ 55 and see what happened.

There are several big hills. As I topped the hill behind West Union, it was at 16. However, the farther I came the better it got. By Harrisville it had topped 20, and believe it or not under the I-77 underpass in Parkersburg (55 mi) it was setting 24.2!! I went on south on 77 to Ravenswood, and it was at 24.3!!! However, I sat in a Dairy Queen drive-thru for 10 minutes and it fell 2 mpg. I then drove on to Pt Pleasant and after 120 miles total (the last 30 on two-lane) and 10 minutes idling not moving, it was still back up to 23.6!!!

I could not believe that holding it down to 55 made that much difference. I have to tell you that I got some mean glances on the interstate, but to get 6 mpg better that at 70, I can put up with it.

LJ

[r gomez]

my last duramax was an 01 extended cab short box 2wd and averaged 17-18gph in 14 mile round trip to work with 16 stop lights 19-20gph highway when pulling 11000 lb trailor got 13-14gph all hand figured the duromax i have now is a 04 extended cab short box 4wd same driving as the 01 i get 13-14.5gph city 16-17 highway and 11 pulling the trailor city or highway (i can find all kinds af stuff to beef up the truck and ger worse mileage does anyone know what i can do to improve mileage

[rob@rone.ca]

Note to LJ

I had that happen to me once - going 85-90 KMH (= 55 mph) on a road which started uphill, and then went down the rest of the way my DIC was down to 11.5 l/100KM which would be near 20.5 US mpg. (Mine is 05 LLY 3500)

When I filled the tank, however, the real consumption was nearer 13.8 l/100KM = 17 US mpg.

The DIC seems to give highly optimistic readings, leaning towards the actual current driving more than the real averages especially when recently reset.

I'm still hoping for some improvement after a few more KM's on it.

[B. Lane]

05 REG CAB WT PACKAGE. 2300 MILES ON TRUCK. CRUISE SET ON 70MPG, MANUALLY CHECKED LAST WEEKEND AND GOT 18.3MPG. ALL HIGHWAY. I GET 14-16 AROUND TOWN. PULLED 4K-5K TRAVEL TRAILER IN SUMMER AND GOT 14-16MPG WITH CRUISE SET@65. MILEAGE SEEMS TO BE GETTING BETTER.

[AndyU]

Originally posted by AndyU:
Well, after thorough testing, towing, empty, highway, city, etc, my mileage went down with the Predator. Running empty, I lost about 1.5 mpg in city driving and about the same loss highway. Towing was about the same, maybe even a bit better, but it's hard to tell because of the affects of wind direction, rate of travel, etc which makes it hard to get a good baseline. The Predator caused my DIC to indicate my mileage went way up, but hand calculating my fuel usage revealed that in fact it went down. Very disappointing.

After restoring the factory program I averaged 19.5 mpg on a 550 mile trip last weekend (empty at 70 mph w/ cruise and a tail wind). I got 22 mpg on the last 100 miles (empty, no wind, 60-65 mpg w/out cruise) after refueling. I think that's pretty good.

The truck definitely gets better mpg w/out cruise on, letting the truck slow up a bit ascending hills and speed up a bit going down. Also, if I keep it at 65 or less my mpg is much better. Also no hard acceleration.

I only have 5000 miles. I'm very happy with this truck. :D Quick update. I now have 15,000 trouble free miles on the truck. I added an Edge EZ and I run with synthetic oil. I can now get as high as 24 mpg if I can roll along at 55-60 without any stopping. The norm is more like 22 hwy and 18.5 city, subtract 1 mpg when running with winter blend fuel. The Edge EZ definately gave me at least 1 mpg better, maybe a bit more.

[REDTRUCK05]

Originally posted by Cliffee:
I'm averaging 16.5 with mostly short city trips. 1800mi on OD. 2005 LLY how fast?

[REDTRUCK05]

Originally posted by AndyU:
</font><blockquote>quote:</font><hr />Originally posted by AndyU:
Well, after thorough testing, towing, empty, highway, city, etc, my mileage went down with the Predator. Running empty, I lost about 1.5 mpg in city driving and about the same loss highway. Towing was about the same, maybe even a bit better, but it's hard to tell because of the affects of wind direction, rate of travel, etc which makes it hard to get a good baseline. The Predator caused my DIC to indicate my mileage went way up, but hand calculating my fuel usage revealed that in fact it went down. Very disappointing.

After restoring the factory program I averaged 19.5 mpg on a 550 mile trip last weekend (empty at 70 mph w/ cruise and a tail wind). I got 22 mpg on the last 100 miles (empty, no wind, 60-65 mpg w/out cruise) after refueling. I think that's pretty good.

The truck definitely gets better mpg w/out cruise on, letting the truck slow up a bit ascending hills and speed up a bit going down. Also, if I keep it at 65 or less my mpg is much better. Also no hard acceleration.

I only have 5000 miles. I'm very happy with this truck. :D Quick update. I now have 15,000 trouble free miles on the truck. I added an Edge EZ and I run with synthetic oil. I can now get as high as 24 mpg if I can roll along at 55-60 without any stopping. The norm is more like 22 hwy and 18.5 city, subtract 1 mpg when running with winter blend fuel. The Edge EZ definately gave me at least 1 mpg better, maybe a bit more. </font>[/QUOTE]What level are u running? I average 15mpg at 70-75. Pulling me 5600# travel trailer at same speeds I get as low as 7.9 as hi as 11, pretty ****ty for such a light load. 6500 miles on

[tboy76]

Originally posted by eagle in flight:
My LLY has been getting about 18 mpg all by itself. When I tow a fifth wheel (weight around 14,000 pounds) and travel at 60 mph, I get around 11.5 mpg. Mileage is at 15,000.

[omercer]

I am very disappointed in the mileage. I'm seriously considering trading my 05 Duramax in for the new Ford when they come out next year. I have 22k miles and mostly at 75mph on the hiway. The dash readout says 13.9 ave mpg. That's pretty close. The best single trip was 15.9 mpg. Most of my driving is at ~ 5000ft elevation in Northern Nevada. I got the 15.9mpg in California close to sea level. Do you think that indicates something? Please explain to me why there is such a wide range of mpg. It doesn't make sense to me that there can be an 8mpg range for the same vehicle even taking into account different driving styles.

[JohnC]

Originally posted by omercer:
Please explain to me why there is such a wide range of mpg. It doesn't make sense to me that there can be an 8mpg range for the same vehicle even taking into account different driving styles. Well, I can't answer your question, but I will say that my local driving is pretty rough on mileage. I have a lot of short trips with a lot of up and down hills. I was getting about 13.5 mpg in this cycle with my '95 and I'm getting about 13.5 mpg with my new D-max. The D-max has only about 1000 miles on it and there is the fun factor coupled with the fact that it is a heavier truck, so I'm not complaining.

Also, my DIC appears to be right on the money. Last fill was 18.3 gallons and the DIC showed 18.1 used. Well within an acceptable margin of error...

[mad]

2500 mile report around town driving without towing:
1) Stock I got 16 mpg...
2) Added LDL from FreedomFuel to the fuel and got 17 mpg...
3) Added Magnaflow 4" turbo-back exhaust and now getting 19 mpg...
Later this week I add an AFE Stage2 intake and am expecting to hit 20!
:D

[JD Diesel]

Just took a trip to Los wages. There and back got 17.6 Speed most of the time was 76 mph. We is happy. the old 454 sub best was 12mph. JD tongue.gif

[AndyU]

Quick update. I now have 15,000 trouble free miles on the truck. I added an Edge EZ and I run with synthetic oil. I can now get as high as 24 mpg if I can roll along at 55-60 without any stopping. The norm is more like 22 hwy and 18.5 city, subtract 1 mpg when running with winter blend fuel. The Edge EZ definately gave me at least 1 mpg better, maybe a bit more.

I now have 22K miles on the truck without any trouble. That 23 - 24 mpg at 55 with no wind and no stops, etc has held true, but remember, that's a pretty unrealistic scenario in normal life. I just said it to show an absolute best case scenario. Real world was more like 20 Hwy and 17 city this winter under the best conditions and 2-3 mpg lower when in 4x4 with snow and very cold. Still, I think that's very good.

Somethings I've done so far that helped improve my economy. (1) Got rid of my lead foot. Seriously, just keeping a real light foot and slower speeds added as much as 2 mpg. Watching the DIC helped me learn to change my habits. That said, the following mods got me another 1+ mpg. (2) Edge EZ running on level one. (3) Boost Stick, but only when combined with the blocking plate and finger stick to bypass the EGR system. Boost stick with a functional EGR had fuel economy go down. the boost stick has helped towing, but doesn't seem to effect my economy empty.

The mods definately helped my economy towing, adding as much as 2 mpg depending upon conditions. I can get 14-15 pulling my 34' x 8' x 9' goosneck horse trailer (10,000-12,000lb) just going 50 mph on nice back roads w/out starts and stops. Pushing that up to 65-70 drops it down to about 11mpg and pusing up to 75-80 takes it down to about 9. As a rule of thumb, economy seems to drop about 1 mpg for ever 5 mph over 55.

I will be adding a corsa perfomance exhaust w/ down pipe soon. I'll let you know if that makes any difference after I've had a chance to observe it in action for a while.

[Dannyboy]

I have 2500 miles on the 06 and my highest mileage so far is 19.5. I'm really impressed so far. My avg. since new is 17, but keeps getting better. My father has an 05 lly and the best we ever got with that is 17.8.

I'm really happy with this truck!!!

[REDTRUCK05]

I own a 2005 2500 HD LT crew cab. MPG has been ugly since delivery. I average between 14 and 15 empty and if I am pulling my 5600# travel trailer, I get between 9 and 11

[rob from bc canada]

These poll results are definitely weird.

Has anyone ever pinned down a cause for those who experience such low mileage?

The numbers of them according to the is poll seems high enough to warrant a dedicated search for the problem.

Any mileage poll should show a bulge in the mid range, and not at both ends.

I drive for best mileage, and with the heavy 3500 4x4, I can reliably get a measured average of near 13 l/100 km (17.5-18 US mpg) driving 80% highway averaging 100kmh = 60 mph, 20% local, with a steep hill for 1.5 km up to my house at the end of each trip. That really cuts it down - it does burn 1-2 l coming upo that alone.

If I reset the computer on the highyway after it's warm, I can get down to 12l/100 = 19.6 mpg average over a 100 mile drive which is probably a bit optimistic. My DIC tends to be at least 1 mpg higher (.5 to 1 l/100k low)

With a high 3000 lb camper on, doing 100 on the highway consumption goes up to 18l/100 = 13 US mpg

These figures are not fantastic, but given the weight of the truck when compared to my old K2500 93 6.5 TD Standard shift which weighed about 1500 lbs less, seem more or less reasonable.

All this leads me to believe that either the poll results are off somehow, or a large number of the 05 LLY's have a significant problem, which could be fixed, because they can do better.

[More Power]

The following two forum threads might help, both to explain fuel economy differences people see and to help troubleshoot a low MPG truck.

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=22302
http://www.thedieselpageforums.com/tdpforum/showthread.php?t=9675

Jim

[USA-1]

16-17 MPG mixed 20 MPG highway at 1800 RPMS.
USA-1

[rob from bc canada]

16-17 MPG mixed 20 MPG highway at 1800 RPMS.
USA-1


Would that be per the DIC, or calculated from tank fills, and recorded miles?

I find the computer is 1-2 mpg optimistic.

In any case, for a 3500DRW that sounds pretty good, or absolutely excellent if it's calculated.

By the way, are there any steep hills in your area?? It seems hauling the 7000 or so lbs up hills seems to bring it down quite a bit.

[Finman]

Got 17.4 on a 1900 freeway trip-no load. Get under 10 pulling 8000lb 5th wheeler. 14-15 around town driving.

[REDTRUCK05]

I have a brand new 2005 2500HD with almost every option and I am getting 15.8 around town and 21.9 on the hwy. What are people using to increase fuel economy?
I could only wish to get that!

[EZHAULER]

I consistently get 15-16 mpg around town driving. (Easy with the foot-driving). I do not do much long Highway driving without a load- but with my toyhauler loaded 12K+ I consistently get 8-9 mpg @ 70-75 mph. I got as high as 11 mpg once trying too increase by driving 55-65mph. ALL hand calculated- filling up till I could see it in the neck. I'm going on a 1600 + mile trip next week, First trip after installing upgrades-EDGE Juice W/Attitude monitor & Cat back exhaust. After getting alot of info off the forum EGT/MPG. I'm going to try and use control and see how good it can get. Follow up info in a cpuple weeks.
__________________________________________________
2004, LLY 2500HD,CC/SB 4X4,Edge Juice W/Attitude Monitor
MBRP 4"Cat Back Exhaust,K & N Air Filter,60 Gal Aux Fuel Tank
Sprayed in Bed Liner,Firestone Air Bags,Front End leved W 305/50 R20 Tires

[REDTRUCK05]

16-17 MPG mixed 20 MPG highway at 1800 RPMS.
USA-1
What is that, about 55 mpg?

[DaveMcClain]

I have a Gulf Stream Ultra with the Kodiak chassis, Duramax LLY,and Banks Six Gun, and at 55MPH, I achieve 15.2 MPG! When pushing to 65 MPH, level territory, the mileage drops to about 12.5. Must be wind resistance.

[Kelly T]

Updated - over 25,000 miles now. 19 MPG seems to be working. Bone stock. 2500 HD CC.

http://home.columbus.rr.com/vfrket/Diesel/Diesel.xls

Note my calculated Miles Per Hour (miles for the tank divided by elapsed time (hours) from the from the DIC) to the far right. BTW, I still like my spreadsheet. There are few rows of data where I had to add a couple tanks together.

Kelly

[fourtenposi]

Has anyone ever looked at the corralation between between getting poor fuel economy and what kind of topper/bed cover they have? I pulled off my high rise topper and I have seemed to gain 3-4 MPG. I only have put about 40 miles on the truck since, but the change seems dramatic. Has anyone seen any results like this? I have made no other changes to the truck. I have a 2005 GMC 2500HD EC 4x4.

[rob from bc canada]

I very much doubt the topper has much to do with it. At most I think you would be getting 1/2 mpg variation for wind resistance.

Mileage can vary dramatically depending on many factors such as wind direction relative to travel (if your going 70 against a 50 mph wind, its just like doing 140, and going the other way would be like doing 20)

In such a short distance, relying on the computer, I find mine reads higher near the top of the tank, and lower mid tank.

Load, speed compared to your average, hills, stops and starts, etc all can affect your reading.

18-19 mpg unloaded 60-65 mph with a few starts and stops thrown in seems to be fairly normal.

[troyf63]

I get between 19 and 21 MPG on the highway at 65 MPH. When I get on it though, it declines significantly. I really like the power when I can keep it out of the shop.

[XTeam524]

Mine does good between 55-64 I can get around 18-20 depending on the day, but if I go any thing over 65 it goes down to 16 even 15. I think that is a bunch of crap. Why can a semi pulling 60,000lbs get 6-7 and my pick-up cant get 10 pulling a 40 enclosed trailer that only wheighs 7000lbs.

[almo]

I just bought a 04 3500 D/A 4x4 lly with 53400 miles. Switched to synthetic motor oil & trans oil. AT a nice easy 65 cruse I get 19 to 20 done by hand. To soon for other milage.

[Freightliner3]

The best and most repeatable mileage is 19.4 empty.I live in the farm country of Minnesota,so most of my miles are hiway.Where the 2005 LLY shines is when I tow my skid steer and attachments,about 11000 lbs.In 24000 miles it has kicked out of Over Drive 2 times. I have added the BD manifold update.I have on order the Banks monster exhaust system and the Banks Air Intake.Will see if they make any difference.My main interest is pulling small heavy loads.If I need to pull large loads I use the Freightliner.

[BigRabbitMan]

I have a Gulf Stream Ultra with the Kodiak chassis, Duramax LLY,and Banks Six Gun, and at 55MPH, I achieve 15.2 MPG! When pushing to 65 MPH, level territory, the mileage drops to about 12.5. Must be wind resistance.
What does your rig weigh? I am considering putting a D/A into my 15,000# FMC in place of the original 440-I.

[kantra]

I have only 750 miles on miy 6.6 and it gets 15.2 per gallon and 11.4 towing my fifth wheel camper 13000lbs I figure that is not bad for an engine not even broken it yet

[kantra]

With 1000 miles on my truck I pulled my 14000lb fifth wheel home the other day not knowing the truck was in 4 high when I checked my milage it was 14.4. But in 2 wheel drive its only 11.3 what gives with this??

[Chuck1]

I have been getting 20 to 22 mpg highway and with my 5th on 15 .Its only a 25footer and i can hardly tell its on there .Man I love this truck .I had a 6.5 before this for 10 years never had a problem did a few adds. thermestat high out put water pump and fan I never had it overheat and i live in some major mountians,I tow quite a bit flat deck trailers and my 5th but after reading about it here. I thought best to do the cooling mods. It was a good truck also when I sold it the buyer said what dosn't work on it ? It was a 94 and had a full load and I thought for a minute and realized every thing worked and had never given me a problem.It was a little slower up the hills than the dmax but a really good truck.If my 05 dmax treats me near as well I.ll be happy. And boy with the extra power and the heated seats I'm in heaven !

[Londell]

I have an 06LLY with 26,000 miles no problems with anything so far. Get 24mpg on highway at 55mph at 60mph get 22mpg at 65mph get 21mpg at 70mph get 19mpg. Mixed driving get 18.5mpg. I tow a 36foot fifth wheel that weighs 14,000 lbs fully loaded and get 10-12mpg depending on grade over here in California. Not bad I say.

[Truck Specialties]

Alot of people believe that a programmer, intake and exhaust are all you can do to optimize fuel economy. Just the tip of the ice berg. The LLY has huge potential, in our eyes it gets the best miliage of all years. I just got back from a 3,000 mile trip with my 05 crew cab dually with 285-75-16's AT's(you know the tires everyone says hurts your milieage) I pulled a tall horse trailer bumper pull, weighs 6500 lbs. for 1500 miles Drove 80mph the entire way, averaged 12.5 mpg. Turned around empty drove 80 to 85mph back home, averaged 22.9mpg calculated not relying on the dash. Speedo is corrected for tire size. Gains come from small things all adding up. Stay with programmers, stay away from tuners like Banks and Edge if you want miliage. Get rid of cats and egr systems that flood the motor with garbage air, open up the intake elbow by the turbo, seal up the radiator and intercooler to force the air through, electric fans are the tits! Water/meth injection will keep the miliage up while towing and egts down. Remember black smoke maybe cool, but it won't make power.:eek:

[dannomatt]

2005 C/C L/B SRW 2WD LLY. 37,000 miles.
Back when it still had the 245s on it, I got a real 18-20, with a best of 20.8. (I drive old...1800 - 2000 RPM)
I put 265s on it, and I had the speedo re-calibrated @ Santa Paula (Ca.) Chevrolet. (Excellent Svc Dept)
I promptly dropped to 15-17, with the best being 18.
I feared something besides the speedo had been programmed, so I called the Service Manager. Sensing that the tires were the culprit, he gave me a golden opportunity: They temporarily reinstalled a set of take-off 245s, re-calibrated, and turned me loose. 250 miles later, I was back at 20 MPG. (Of course I had them put my 265s back on - They look better!)
All the MPG numbers are figured old-school - No DIC.

So. Where-oh-where do I go to get that fuel economy back? The local diesel shop recommends the Diablo Sport programmer. As good as 2-4+ MPG is what his customers claim they picked up. He says don't bother with intake/exhaust mods.
At 1800 RPM, cruise control on the freeway, I'm a consistent 17-18, and I know that's not bad...But if there's a little better to be had, I'd be interested in hearing about it.
Thanks

[DmaxMaverick]

Welcome aboard!

Looks like your mileage didn't change, much. Your dealer didn't (and can't) reprogram/correct the speedo/odo. The changes they make in the PCM ONLY correct for ABS operation. The tire change increased your roll-out by about 5%, which corrects your fuel mileage almost back to where it was before, but not quite. While taller tires offer a potential for an economy improvement, wider tread reduces it (for a number of reasons). Switching to a 235/85/16(32") or 255/85/16(33") offer the best opportunity for an increase.

Power programmers won't improve your mileage, generally. They may offer the opportunity to help you improve it, but ultimately, it will be your driving style and habits that will determine the actual outcome. The Predator programmer (Diablosport) will allow you to correct the speedo/odo for tire size changes and increase power, but won't help fuel mileage by itself. Only your right foot will do that. Normally, having more power on tap temps one to use it, which will in every case, decreases mileage. The advantage of having more power allows you to drive under the same conditions in a higher gear (under load or ascending a grade), which can increase economy, provided you resist the temptation to use the additional power. If you get/use any power programmer, be sure to also install, at the very least, an EGT (Exhaust Gas Temp) gage. A boost gage is also recommended, but the EGT gage is necessary to monitor the exhaust temp, which can cause sever engine damage if exceeded, or prolonged.

[dannomatt]

Thanks. I'll continue researching. It seems that gauges are are universally recommended to go along with most modifications. Understood.
It's frustrating how many opinions there are regarding the re-programming of a vehicle's ECM.
The dealer who re-flashed mine insists that all they did was correct for tire size. (They also helped me determine that the tires were the main culprit in the loss of MPG)
The local diesel mechanic says the dealer unavoidably gave me some EPA / AQMD 'updates' that are probably causing additional fuel consumption. (Dealer insists that only speedo 'files' in ECM were opened)
A GM rep told me that the only ECM re-flash they support is not related to tire circumference. He even went as far as saying "...there are some parts of your ECM that are better left un-touched."
The 2-4 MPG loss isn't killing me, especially considering that I know the tires are most of it. My goal now is to determine if there are mods that can some of it back.
Great truck otherwise!
Dan

[DmaxMaverick]

Some mods can help improve your mileage. Problem is, at what cost? The reality is you may not see an ROI (Return On Investment) until 100-200K miles or more, or never (depends on how much you spend, the mileage increase, and how long/far you keep/drive the vehicle). Driving habit adjustments are free. However, in every case, YMMV.

[Diesel 4me]

Wishing that mileage could be improve don my Dmax as well. I have a 2005 with aprox 22,000 miles - stock sized tires and my DIC records aprox 13.8 while driving down the highway at 60 (Ca).... Engine stock for now ... no programmer :rolleyes:

[JohnC]

Don't believe the DIC unless you have confirmed its accuracy. Calculate it based on mileage and fuel purchases. (Verify odometer accuracy, too....)

One easy way to check the accuracy of the DIC is to zero the fuel used when you fill up and see if it agrees with your next fill. Repeat a few times to get a feel for its accuracy.

[asm]

I voted less less then 15MPG.

My Duramax is a 2004 Dually with crew cab. When towing a 11000# fifth wheel, going up 395 to Bishop in speed between 35MPH to 55MPH, I get 9.8MPG. 35MPH is not by choice, as LLY is known to overheat when push hard going up a long grade.

I got my best MPG yesterday going from Bakersfield to Orange county to drop off a trailer. The trailer weight about 7000# and I kept the speed between 55 ~ 65MPH. Of course, the returen trip was driven with nothing in the back of the truck. I actually got 14.1MPG. Not as high as some of the 20MPG claim I see here, but much better than 10MPG I used to seeing.

[AKMark]

05 LLY, CC, LB 94K miles, 15K hours, unknown age of injectors, runs great though.

In Alaska with temps in the -20 range, 15 driving approx 14 miles one way twice a day every day. (56 mph, if I drive 60 this drops to 13)

17 over a 85 mile drive with 3 hills over 8 degrees of upward angle. (67 mph)

This is with #1 Diesel, mixed with 2 stroke oil.

I haven't taken it on any longer road trips yet, and unfortunately all of our roads out of Fairbanks will encounter big hills.

[gary_lucas]

Had the truck for almost a year now, put about 12k on it.

Overall thinking it's a great truck, but the fuel economy is less than I hoped for.

My last set of numbers put me in the 14.2mpg range with mostly city driving unloaded.

When I got the truck last year it had:
* Modified exhaust (Not sure what it is, going to find out tomorrow).
* Edge Juice with Attitude.
* p265/75's

I've removed the attitude module this evening for the first time. I had a bakflip installed this past week and I'm going to get a Kennedy tuned ECM for our trip down to Las Vegas next month. We'll be hauling a 3200lb trailer so I'll get a good idea of the mileage at that point.

My understanding of how to drive with fuel efficiency in mind basically starts and ends at takeing it easy, don't floor it to get up to speed, Keep it at the low end of the speed spectrum (IE on the highway keep it towards 55 rather than 70 if at all possible...)

When the current 265's start getting old I'll probably replace them with the stock 245's and hope to see some better fuel economy.

[gary_lucas]

Followup, just drove from Vancouver BC to Las Vegas Nevada and back up the I-5 in California.

Combined average of about 17.5MPG the whole way. I was towing a 3000lb trailer with a really low profile (low wind resistance).

I was getting close to 19MPG on the I-5 on the way back, going through the mountains brought it down to 17.5 approximately.

[AKMark]

First long road trip with my 05 this last weekend.

734 miles driven (per gps). I have an 80 gallon external tank in the bed that keeps the main topped off. It took 35.7 gallons to top off the external after we got home and I confirmed the main was full.

20.7 mpg, I drove 67mph (per gps) except for about 200 miles on the way back when I stepped it up to 72mph to keep up with a convoy of friends.

Considering my motor has over 16K hours on it, I'm happy.

[dfenze]

04.5 LLY with 110K miles. When new I was getting 18-19, no matter what I did. now, I get around 15-17. I have some upgrades, but I am also having a lot of issues with the motor right now.

[AKMark]

Tried a different fueling place. Glad I did.

Running short trips (14-18 miles at a time usually) I'm averaging 18-20 with summer weight fuel and a little bit of 2-cycle oil to give me some lubrication.

I haven't had a long road trip since using this stuff, and my bad luck I will be back to winter fuel before long. Summer doesn't last long here.

I found two of the other stations I used to fill at run winter fuel all year round. Jerks.

[Rhynno]

I have had my 2005 GMC 2500HD Duramax for over a year now, and it gets horrible fuel mileage. It is lifted with 35" tires, has an aftermarket air intake system, Edge Attitude chip, and aftermarket exhaust (not sure on the gears, assuming stock). I get between 10.5 and 11.5 MPG. At first, I thought it was the chip because it was set to Stage 3 and had tons of power, so I changed it to Stage 1, the economy setting. I only gained .5 MPG, so I changed it back to Stage 3 to get the 70hp and the 120 Ft. lbs. of torque. I also have a 2000 Ford Excursion 7.3 Powerstroke which I have owned for 14 years. It is much heavier than my Duramax, especially when the 44-gallon fuel tank is full. It has a 6-inch lift, is running 37" BFG KM2 M/T tires, and the gears are stock. For the life of the Excursion, I have averaged 17.3 MPG around town and I have even been able to achieve 20.4 on the highway, loaded down with camping gear and people. The big difference I see between these vehicles is the Excursion has hubs and the Sierra doesn't. I haven't had a lot of time to work on the Sierra, so I am sure I will be able to tweak it some once I can spend a little time and money on it, but I was hoping it would at least deliver 15 MPG around town. I hope to get one of the hub conversion kits from the Ram Man this year and get that installed to see what difference that makes, but at this point, I have just done routine maintenance. I will also give it a tuneup when time allows. Love the truck though, drives awesome, very comfortable and get amenities!

[wmholden]

In 2005 I averaged 21.7 mpg on a trip from Texas to Savannah @ 67.5mph. Last week I averaged 18.5 over an 800 mile trip, half of which include about 1200 lbs of hickory in the bed and 70-75mph. So I have no complaints. I love my LLY. And since I now live in town(again)my city mileage is 13.5 to 13.9 mpg. We are looking at the DT400 performance package now. We are approaching 160K and are ready for mods.
DT400 components for under $800 total look like a nice foundation for performance- now and future.:) I hope to find reviews on the DT400 kits