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  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)
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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
    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

  10. #10
    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.

  11. #11
    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.

  12. #12

    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

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

    Default New hope!

    After over a year of complaints to my dealer, and many letters to GM about my UGLY MPG, I did get help! The general manager of my dealer I bought my truck from called me in. He had me uninstall my edge system and GAVE me a hypertech! After install I got 20.5 at 70 and 18 city! Keep in mind that these are computer reads and not yet calculated. With my past calcs, the DIC has never been off by more than 1.5 MPG. This would put me at an increase of between 3 and 5 MPG better across the board! I will keep you informed on later calcs.
    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

    Default

    I'm sure a lot of us will be waiting to hear further results from that installation.

    Not knowing much about these things, I wonder what programming they put on that, that they couldn't put into their own PCM.
    2005 Chev K3500 SRW Dmax Allison Ext Long Box

  15. #15
    Join Date
    Mar 2000
    Location
    Loyal WI US
    Posts
    10,792

    Default

    Quote Originally Posted by REDTRUCK05
    After over a year of complaints to my dealer, and many letters to GM about my UGLY MPG, I did get help! The general manager of my dealer I bought my truck from called me in. He had me uninstall my edge system and GAVE me a hypertech! After install I got 20.5 at 70 and 18 city! Keep in mind that these are computer reads and not yet calculated. With my past calcs, the DIC has never been off by more than 1.5 MPG. This would put me at an increase of between 3 and 5 MPG better across the board! I will keep you informed on later calcs.
    By nature, chips and tuners alter fueling by sliding fuel to the engine under the table. This will cause the DIC to become less accurate in most cases. The severity of this inaccuracy will vary depending on where the changes have been made. The Edge for example, does very little to the cruise fueling and is more of a demand based unit. The Hypertech is not particularly aggressive down low, but could skew the readings. My Economy tunes have varied in their response to DIC calculations. Some have been spot on and others have skewed it. Best is to hand check it.

    My Dyno plot shows the power output at full pedal:





    Isn't pretty...
    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.
    2016 GMC K3500SRW D/A CC short LTZ

    Custom tuning in house using EFI Live tuning software!

  16. #16
    Join Date
    Aug 2005
    Location
    HUGO, MINNESOTA
    Posts
    126

    Default Stacking Chips

    Because with the hypertech, I have lost the the monitor, can I reinstall my edge along with the hypertech? Is this dangerous?
    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

  17. #17
    Join Date
    Mar 2000
    Location
    Loyal WI US
    Posts
    10,792

    Default

    That one you should be OK with, but only because it is an LLY. The LB7 Juice still adds timing.
    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.
    2016 GMC K3500SRW D/A CC short LTZ

    Custom tuning in house using EFI Live tuning software!

  18. #18
    Join Date
    Nov 2007
    Location
    Tulsa, OK
    Posts
    7

    Default

    What setting is your Hypertech on in order to get the MPG? I have one and mine has worsened. I have tried stage 2 and stage 3, I am going to set stage 1 today. I have had this a week. I like the response and the increase in HP. My DIC says 13.7 but calculated it is more like 12 to 12.5. In either Stage 2 or Stage 3. Without the Hypertech my DIC is accurate. I will post MPG's after I run it in Stage 1 for a couple of days. As far as driving habits go, I do not drive this like a race car. Coast to a stop for minimal use of brakes and ease into accelaration when taking off or merging. Before I took it to the dealer a couple of months ago I was getting 16+ without anything. I want just that back at least. They had to update my #7 injector pigtail. Now I have a couple of codes I will also post that later as well.
    '05 CC 4WD "Duallie" LT Loaded D/A
    '94 7.3 Turbo, not PSD Ex-Cab 5 spd "Duallie"
    '03 Silverado Ex-Cab 5.3
    '66 Ford Short Bed 6/3 on the tree

  19. #19
    Join Date
    May 2012
    Location
    Langley
    Posts
    369

    Default

    I just got a new (to me) 2004.5 Sierra 2500 Diesel.

    Truck has 181,000k on it.

    We took it for the first trip with the camper on the highway this past weekend.

    It looks like I was averaging 15.6 to 16.4 l/100k (15.08 / 14.34 MPG)

    Looking at instant fuel economy I could see it was hitting as low as 9l /100k on flat stretches at constant speed. Our trip was only about 70k on the highway at constant speeds, so I expect to see better results when we go to Tacoma (300ish K)

    Mileage in town is more like about 17-18l / 100k (13MPG)

    It's BC so it's not flat, but it wasn't terribly hilly either, we were towing our TT (3000lbs current weight).

    I was not driving as efficiently as I could, we're going to Tacoma in 2 weeks, I think that will be a better test (flatter) and I'll try to keep the lead out of my foot... And the drive is quite a bit longer... So I'll get a better sample...

    My truck came with an Edge CTS in it, but it looks like the firmware hasn't been updated for awhile. It's missing the mileage calculator / mileage coach that should be there in theory.

    I'm going to bring it upstairs and see if I can get it updated this weekend. I've only used it at level one, which theoretically is a mileage setting.

    Any suggestions? Looks like someone already updated the mufflers, there is two giant tailpipes, I'm pretty sure that's not stock.

    Thanks all

  20. #20
    AKMark is offline Building another 6.2L powered vehicle
    Join Date
    Dec 2006
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    North Pole, Alaska
    Posts
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    Default

    I picked up an 05 2500HD CC LB 4x4 this summer. It was used on the north slope oil field of Alaska and has been prepped for extreme weather. The bumpers on it must weigh about 300 lbs a piece. It only has 91K miles on it, it does have over 12000 hours thanks to excessive idling trying to keep the occupants alive when temps dropped below -60.

    Love it, TONS of power and does everything I ask of it and more. I had almost 3000lbs in the bed a few weeks ago and the rear suspension wasn't sagging, it was even with the front, but not sagging. The traction like that was amazing.

    Pulling a Dead 6.0L Powerstroke she out accelerates my 6.5 empty.

    I have a 13-15 mile commute everyday depending on which entry point to the Army base I have to use. I averaged 15-16 doing this run every day this summer. It dropped a little when I was pulling, but not much.

    I haven't had a chance to take her on a longer drive to see what it does when it runs a full tank of fuel without having to warm the engine up multiple times. With it being -30 now and running it a little more than normal mileage is still 14, which is better than the old 6.5 did.

    The 6.5 Burb got 14 in the summer doing the same run and about 12 in the winter. On the Hwy the Burb pulls close around 18-19 empty and about 13 pulling a K-5 on 1 tons and 36's.

    Would love to find a way to get into the 20's, but my 235/85's probably aren't helping. I'll live with the lower mpg's to run these tires. Oh and I hand Calculate my mileage from a Garmin GPS, as both the Burb and Truck's speedo's are slightly off thanks to the tires.
    05 2500HD CC LB LLY, 4x4, 3.73s 235/85R16's, webasto cab heater, to keep it warm.
    03 Buick Rendezvous - When you average over 80 miles per day driving around, you need one of these.
    85 K-5, 6.2, SM465, Rockwell T221, 1 tons, 36's. More goodies to be installed as time and money allows.
    82 K20, 6.2, SM465, NP208, stock except for bed rack, snow plow, and glow plugs are on a toggle switch. It works great for plowing!
    72 Postal Jeep - Yet another project

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