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Thread: Maximizing LLY fuel mileage

  1. #1
    Join Date
    Jan 2005
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    St. Louis, Missouri (pronounced misery)
    Posts
    203

    Default Maximizing LLY fuel mileage

    I am new to this forum, and I have to admit new to Diesels.

    My father had had his '93 6.2 since 95. It has been and continues to be a great truck. He is consistently getting 22-24 MPG HWY running 70MPH.

    I have an '06 crew cab 4x4 with only a thousand miles on it. I have the LLY motor (darn it!), and it is bone stock. The only change or addition is a Line-X spray in Bedliner. I have stock wheels and Tires, no bug deflector or ANY changes to the truck. So far, I haven't gotten better than 16 unloaded on the interstate at 65MPH. I think I am running the 3.73 gears in the rear, but I don't know how to tell for sure (can anyone tell me how to tell?).

    I am sure that as the motor breaks in that I can expect it to get a 'little' better. What is 'average' for most LLY users? I have read several threads, and mine seems low.

    I bought the truck for the power, and pulling. But, the MPGs attracted me too. Now I'm not really seeing them. Sorry for being so long winded, but this brings me to my real question.

    Would the addition of an exhaust system and possibly a cold air intake increase my mileage? The added horsepower would be good, but I am most concerned about mileage. Also, I pull my 6K travel trailer a couple of times a month. So towing mileage is sort of a consideration too.

    I am sort of a MPG nut, and would like to see a little better than 16. My 1/2 ton Chevy with 5.3 gasser got 18 MPG REGULARLY at 70-75 on the interstate (Really!)
    Scott
    St. Louis, MO


    '06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
    '99 BMW K1200LT "Special K" my other "SUV"
    '07 Salem by Forest River Fifth Wheel Toy Hauler

  2. #2
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Arrow

    Hi Scott,

    You've asked an important question, regarding fuel economy, and your truck's apparent lack of it.

    First, all Duramax equipped 2500HD/3500 series trucks come with 3.73 differential gearing. The new 6-speed Allison with its double-overdrive will produce engine rpms roughly equivalent to a 5-speed Allison equipped truck with 3.42 gears. I assume you're running in 6th gear while on the Interstate.

    The thread we began a few weeks ago describing a Scatter Plot would be a good place to begin when troubleshooting a low mpg truck. This requires an EGT gauge, and it might be worth installing as a first step (along with a boost pressure gauge). (Read the whole scatter plot thread - to get most out of it.)

    We have a pretty good understanding of the relationship between EGT, boost pressure and fuel economy. It requires burning a certain amount of fuel to achieve a given speed. Exceptionally low mpg's must mean either your truck is producing higher than normal drag or the engine/drivetrain isn't running efficiently. Again, an EGT/Boost gauge would show that.

    For example, a low boost pressure (due to some turbo or turbo control issue) would cause higher than normal EGT's and allow the truck to produce more smoke. You'll see this immediately with gauges.

    IMO - The 2006 LBZ ought to be 20 or better when driven unloaded at 65-mph.

    Jim

  3. #3

    Default

    Quote Originally Posted by Stlheadake
    I have an '06 crew cab 4x4 with only a thousand miles on it. I have the LLY motor (darn it!), and it is bone stock. The only change or addition is a Line-X spray in Bedliner. I have stock wheels and Tires, no bug deflector or ANY changes to the truck. So far, I haven't gotten better than 16 unloaded on the interstate at 65MPH.
    In spite of a billion posts to the contrary, that's exactly the mileage I see with my '02 LB7 truck and have since Day 1.

    I got ONE tank at 20.02 but I had the cruise locked at 55mph as a test. It ain't worth it.

    I just don't see how other people are getting 18-20mpg at 70mph in the same truck as I have.

    I think I am running the 3.73 gears in the rear, but I don't know how to tell for sure (can anyone tell me how to tell?).
    You've got the 3.73 gears because that's all the diesel truck is available with.

    - Mark
    '02 Sierra 2500HD D/A 4x4 CC SWB, Kelderman Air Ride rear suspension, Bilsteins, Westin step bars, Hawk HP SuperDuty brake pads, Tekonsha Prodigy, Line-X, 265/75-16 Michelin LTX M/S -- Truck Pics

    Race car:
    http://www.justracing.com/homepage/mdadgar

  4. #4

    Default

    Is it a Dually? (I think most use DRW in their sig for that), and is it a 3500?

    These add wind resistance and weight.

    Also, do you not have the higher ratio 6th speed overdrive tranny? If so you should be getting better than my 05

    Is your mileage calculated from ODO readings and tank fills, or is that what the DIC says? My DIC is always 1-2 mpg optimistic.

    In any case that mileage seems quite low, unless you're climbing a lot of hills at 65. I find with the weight of these things, they really suck up the fuel on hills.

    Driving empty with mine (also bone stock) I get calculated averages of 17.5 US mpg driving mostly highway at 60-65 (with quite a few traficc lights), some local driving, and some medium hills mixed in. I record every fillup I put in it.

    This has not changed for better or for worse that i can see since new, I now have about 8000 miles on it.

    On straight flat highway at 65 I can get around 18.5-19 US MPG
    2005 Chev K3500 SRW Dmax Allison Ext Long Box

  5. #5
    Join Date
    Feb 2006
    Location
    Trenton, MO
    Posts
    106

    Default

    Don't expect to much better until you get more like 5k to 10 k on the odo. These duramaxes take a while to break in depending on your use. I drove about 2.5 tankfulls here a while back strictly highway and I averaged 18.8. And that was hand calculated with adding 2.5% for my 265s. BUT I tried to keep it at or under 2k rpms. I don't think people are going to see a drastic differance in the 6 speed ali fuel mileage at lower speeds... rpms. compared to the 5 speed ali. I also think using a good fuel injector cleaner as in FPPF Total Power on every tank helps mileage approx. 1 mpg. Just my experience with my 05....
    Dad's truck
    02 GMC Crewcab Dually Duramax Allison 4x4
    Amsoil Ea Air Filter
    Post OEM Mega Fuel Filter;
    FilterMag on Fuel & Oil;
    Fumoto Drain Valve
    Kennedy Diesel Performance ECM
    Gauges; Kennedy Diesel Lift Pump
    Sheffenecker Mirrors

    Sold
    05 GMC Crewcab Shortbed Duramax Allison 4x4...

    Dad's truck also (My daily driver)
    2007 Classic Crew Cab 3500 Dually
    Kennedy Diesel Mega post Fuel filter
    Kennedy Diesel Lift pump & EGR blocker Package & fingers stick
    Bilstein shocks
    Amsoil synthetic oil and filters
    FilterMag on fuel and oil
    Fumoto drain valve
    Amsoil Dealer

  6. #6
    Join Date
    Jan 2005
    Location
    St. Louis, Missouri (pronounced misery)
    Posts
    203

    Default milage issues

    Thanks for all the replies. I WILL jump in and read ALL on the scatter plot thread. I DO have the 6spd Alli, and I DO run in overdrive. The truck is a 3/4 ton not a dually.

    I just bought a case of stanadyne blue label cleaner, demulsifyer, lube, cetone, etc... I wonder what difference that will make. I haven't used it yet, I am waiting on the O'Reilly cetan booster and lube I used to run through.

    I DON'T have a heavy foot, I run 70 mph unloaded and I take my time getting up there. So my mileage seems quite low to me. In all fairness though, I have not hand calculated what I am getting. I am relying solely on the DIC. This may be faulty.

    So back to my ORIGINAL question: Will adding an exhaust and intake kit improve milage? I have read where some guys have even put in a performance chip and gotten as much as 5mpg (they claim) better than stock. Is this really true?

    It seems that modification would pay for itself in about a years time!

    I wanted thoughts from the experts before I plunk down the coin for the toys.
    Scott
    St. Louis, MO


    '06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
    '99 BMW K1200LT "Special K" my other "SUV"
    '07 Salem by Forest River Fifth Wheel Toy Hauler

  7. #7
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Lightbulb

    By all means, hand calculate the mileage. Try to use the same pump, and use the pump the same way each fillup. I normally just set the pump on the middle speed setting, and remove the nozzle once it kicks off.

    A larger exhaust system and free-flowing muffler can have a small effect on fuel economy - but not the 3-4 mpg you're looking for. I'd get gauges first....

    Jim

  8. #8
    Join Date
    Jan 2005
    Location
    St. Louis, Missouri (pronounced misery)
    Posts
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    Default Following your suggestions

    Quote Originally Posted by More Power
    By all means, hand calculate the mileage. Try to use the same pump, and use the pump the same way each fillup. I normally just set the pump on the middle speed setting, and remove the nozzle once it kicks off.

    A larger exhaust system and free-flowing muffler can have a small effect on fuel economy - but not the 3-4 mpg you're looking for. I'd get gauges first....

    Jim
    Thanks Jim, you and everyone else on this page are a true asset. I am going to order the guage kit from Kennedy first I guess. Othere than the pyrometer and boost, is there something IN a gauge that I should be looking for?
    Scott
    St. Louis, MO


    '06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
    '99 BMW K1200LT "Special K" my other "SUV"
    '07 Salem by Forest River Fifth Wheel Toy Hauler

  9. #9
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Wink

    Gauge brand, model and mounting location are largely personal choices.

    Have a look at the 2005 Gauge Update to get an overview of what's available for conventional gauges and what sort of mounting appeals to you.

    Personally, I like the dual gauge pillar pod for boost/EGT, mainly for visibility - and they look cool there. The Edge Attitude comes in a close second, though I don't know about 2006 model year requirements. The vendors will know....

    Jim

  10. #10
    Join Date
    Feb 2000
    Location
    Houston, Texas, USA
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    Default

    My 2003 and now my 2006 seem to be pretty close in terms of mpg. From what i've seen driving diesels for a number of years.... anyone claiming 20's at 70-75 is flat out fibbing. Or they are going downhill with a tailwind.
    David Proske #418
    '06 CHEVY LBZ 2500HD CC LWB
    '03 GMC 2500HD D/A *sold* :-(
    '94 K2500HD W/GV OD

  11. #11
    Join Date
    Aug 2002
    Location
    Fort Benton, MT
    Posts
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    Default

    I have to agree with David. The only way I could get 20 mpg out of my '02 is if I drive like I'm following old folks to church. On long trips on flat ground with no wind and cruise on 75 I get about 19. Most of the time 17-18. Less in the wind or if I'm in a hurry.

    I have the Edge box and a 4" exhaust. I can't say that they really made much difference in economy. Maybe a little. Towing milage is better than stock if you pull the hills at the speed you would have stock.
    2002 Chev D/A CC LB - it's gone

  12. #12
    Join Date
    Apr 2002
    Location
    northern IL
    Posts
    5

    Default Improving Fuel Mileage

    The cost of fuel has modified my driving a bit. Last weekend a 700 mile trip was done mostly with cruise control and 65 mph (slower than I usually drive). The tonneau cover was in place as a couple of threads suggest this does better than my tailgate down theory. A calculated 21 mpg was about 3 mpg better than my normal (lightly loaded (weight) and standard drag profile) . Lower speed and tonneau did make a difference.

    Gizmo
    2002, 2500HD, 4wd, extended
    Stock Duramax/5 speed Allison

  13. #13
    Join Date
    Aug 2005
    Location
    HUGO, MINNESOTA
    Posts
    126

    Default 05 Lly Owner

    Quote Originally Posted by Stlheadake
    I am new to this forum, and I have to admit new to Diesels.

    My father had had his '93 6.2 since 95. It has been and continues to be a great truck. He is consistently getting 22-24 MPG HWY running 70MPH.

    I have an '06 crew cab 4x4 with only a thousand miles on it. I have the LLY motor (darn it!), and it is bone stock. The only change or addition is a Line-X spray in Bedliner. I have stock wheels and Tires, no bug deflector or ANY changes to the truck. So far, I haven't gotten better than 16 unloaded on the interstate at 65MPH. I think I am running the 3.73 gears in the rear, but I don't know how to tell for sure (can anyone tell me how to tell?).

    I am sure that as the motor breaks in that I can expect it to get a 'little' better. What is 'average' for most LLY users? I have read several threads, and mine seems low.

    I bought the truck for the power, and pulling. But, the MPGs attracted me too. Now I'm not really seeing them. Sorry for being so long winded, but this brings me to my real question.

    Would the addition of an exhaust system and possibly a cold air intake increase my mileage? The added horsepower would be good, but I am most concerned about mileage. Also, I pull my 6K travel trailer a couple of times a month. So towing mileage is sort of a consideration too.

    I am sort of a MPG nut, and would like to see a little better than 16. My 1/2 ton Chevy with 5.3 gasser got 18 MPG REGULARLY at 70-75 on the interstate (Really!)
    I own a 05 LLY 2500 HD crew cab. My MPG pulling 5600 LBS at 65 mph has been between 9 and 11 mpg. the cooling fan runs at any temp above 70 F.
    2007 GMC 2500 crew cab LBZ
    6.6L duramax
    Roof markers and XM radio
    Rhino liner
    Plat series nurf bars
    Putco bed rails
    Truxedo bed cover
    edge juice W/ monitor temp uninstalled.
    Hypertech stage 3

  14. #14
    Join Date
    Mar 2006
    Posts
    44

    Default

    '04 1/2 LLY/Allison 4Door short bed. First tank got 17.4 MPG. I now have 32k on the odometer. 42 miles each way to work. I run 68-70 on the cruise control. 19.4 calculated last tank. Tire pressure 60 all 4 wheels except when towing, then 80 on the rear.

    8000# travel trailer, 4 adults in truck 500# in bed, 11.3 calculated at 65-70 on cruise, in west Pa.

    After initial breakin in I averaged 18.4. I recently had EMC reflashed due to smoke at idle. Fuel mileage is up a little, say .2mpg.

  15. #15

    Default

    Also, I would hand calculate your fuel mileage as a longterm average over weeks and months, not single tankfuls. A spreadsheet is a great way to do this. Just a moments worth of data entry, or enter the information off your fuel card statement each month. My Holiday and Shell cards record mileage at the pump at the fillup, so its really easy to do by the month. I know when a particular trucks performance has changed, usually a dirty air filter is to blame.

    You'll soon settle into a number that will be noticable when you change something like intake/exhaust, etc. When you complete the mod, start another longterm average over a number of weeks and tankfuls. Then and only then will you know if you have truly picked up a solid increase in MPG.
    2011 Chevrolet Tahoe 5.3L daily driver
    • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW, '05 Denali
    • Total GM diesel miles to date : ~950K

  16. #16
    Join Date
    May 2006
    Posts
    8

    Default 2005 DMax 2500 Crew

    I ran a trip Minnesota to Kentucky. A hair under 1900 miles all freeway cruse on 70-75mph. I have a Hypertech tuner and ran it in stage 3. 17.4 MPG hand calculated. Not impressed. Would have got 21-22 with my 02. Black smoke when you put the wood to it and alot of it. Anyone have any thoughts on that? Does not seem to smoke when running in stock.

  17. #17

    Default

    My Hypertech tuner is great at adding fuel (power) but not so great at adding enough boost to burn all that extra fuel (black smoke).

    If I were running down the road unloaded and wanted max mileage, I'd either add enough boost to burn the extra fuel, or return the truck to stock settings for the trip.

    Your speed of 70-75 has alot to do with it. In fact, I'm a bit suprised that your LLY did that well!

    Next trip try stock settings and see how you do, or call JK and add a BoostStick and enjoy the extra (although probably unused) power...
    2011 Chevrolet Tahoe 5.3L daily driver
    • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW, '05 Denali
    • Total GM diesel miles to date : ~950K

  18. #18
    Join Date
    Feb 2000
    Location
    Michigan
    Posts
    365

    Default 20 mpg is possible at 70mph

    This past winter I made some trips to snowmobile and got 20mpg at 70 mph hand calculated (on winterized fuel, too). I filled up the truck, hit the expressway, drove a couple hundred miles to our destination, and filled truck up. I did this 3 or 4 times. No fibbing here...


    Quote Originally Posted by David Proske
    My 2003 and now my 2006 seem to be pretty close in terms of mpg. From what i've seen driving diesels for a number of years.... anyone claiming 20's at 70-75 is flat out fibbing. Or they are going downhill with a tailwind.
    2016 GMC SLT 2500 CC 4x4 Duramax All Terrain 59K
    1995 GMC 2500 EC 4x4 6.5TD,236K,Custom Tuning,HX40W-II turbo,Amsoil Bypass Filter,KD Exhaust,FSD Cooler,KD Headlight booster,Hi-cap Cooling
    TDP member #14

  19. #19
    Join Date
    Mar 2000
    Location
    Loyal WI US
    Posts
    10,792

    Default

    I can't get 20's at 70 mph, but at 60-ish mph I can run solidly into the 20's. It will be interesting to see what Uncle Wally reports as I just sent him a tune for his dually. The DIC shows encouraging results so far, just need to hand check to verify...
    Kennedy Diesel-owner
    More than just a salesman-I use and test the products that I sell on a daily basis!
    Superflow Lie Detector in house
    2002 Chev K2500HD D/A CC Long LT 11.77@ 124mph at 7700# fuel only-e.t. needs help
    2005 Chev K3500SRW D/A CC Long LT(SOLD)
    2007 Chev K2500 Classic EC Short LT (Sold)
    2012 GMC K3500SRW D/A CC Long LTZ Happy Birthday to me! Built 1 working day after my birthday and delivered 7 days later.
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    Custom tuning in house using EFI Live tuning software!

  20. #20
    Join Date
    Nov 2002
    Location
    Howard Lake, MN
    Posts
    381

    Default

    Just to give a comparison as to what my fuel mileage was before John's tune.

    05 LLY Crew Cab Dually - stock tires.

    Loaded - light to moderate

    Modifications at the time were John's 4" exhaust, Uni Air Filter, Mega Filter and Lift Pump/Fuel Pump Pickup mods. I had EFI Live at the time, but chose to leave the stock tune in it for a while.

    I was pulling a two place snowmobile trailer with two snowmobiles on it and I had one snowmobile in the bed. There were three of us and our gear in the truck. We added two honda three wheelers to the trailer on the way back.

    I drove from Minneapolis to Island Park Idaho in March this year. Temperatures were in the mid 40's for most of the trip out and in the low 60's for most of the trip back.

    My best fuel mileage per tank - hand calculated - was 12.5. That was treating the throttle like an egg, using cruise at 60 and never letting the tack go over 2000 rpm.

    My worst fuel mileage per tank - hand calculated - was 10. My personal belief is that I got a batch of marginal fuel. I didn't change my driving style much, so I don't know of any other explanation.

    Unloaded - with tonneau cover - EFI Live now installed - only changes to the tune were to EGR controls. No added fuel, no altered timing.

    I get my best mileage at 50-52 mph. Treating the throttle like an egg, never letting the tack over 2000 rpm even on acceleration. I can get between 16 and 18 mpg. I drove this entire tank of fuel - no wife variables on this one.

    I usually don't drive that way day to day, nor does my wife. Normal driving yields a pretty consistant 14-15 mpg.

    The only issue is that I spoiled my wife with the new truck, and she won't drive our gas miser car anymore. My wife drives totally different than I do even with me giving her constant grief. Her fuel miser skills have gotten better, but are still not at my level. She only understands two pedal postions: foot completely off the pedal, and the pedal all the way to floor (for both foot feed and brake pedals). With that said, it's hard to get a good fuel economy run when my wife drives. I will work on that when we head to Pittsburgh in July. That way I can actually show her what I am talking about.

    Sorry for the long post, but I thought it would be good for a comparison. I installed John's tune yesterday and drove about 45 miles on it. Fuel mileage on the DIC was astonishing. I won't give numbers at this point as I need to hand calc it. You'll here from me after a couple tanks of fuel.

    Waldo
    05 K3500 D/A Crew - Sport Red and sexy! JK exhaust and filtration, guages and EFI Live. What a blast!

    71 K20 awaiting Cummins 12v swaps

    For Sale: Deep Sump Trans Pan for a 4L80E

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