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Thread: LMM Fuel Economy

  1. #61
    AKMark is offline Building another 6.2L powered vehicle
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    Those tires are pretty heavy, that makes a bigger difference than most realize. Also, how far off is your speedo now? Make sure to take that into consideration.
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  2. #62
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    The lift kit won't affect as much as the tire size, With 373 rears and adding bigger tires the allision computer starts changing things, sensing the load from the tires it adjusts to more of a tow mode, changes the shift points and converter lockup.
    I know of a truck with 22.5's that won't go into full over drive lockup untill 25mph because of the matchup of tire to rear end.
    As always it takes a diesel a long while to break in compared to gas. It took about 17000 mi to notice a dif in my cat330

  3. #63
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    Jun 2008
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    Quote Originally Posted by craigth View Post
    If I were to do it over again, and especially with the $2.99 for 3yr gas purchase promotion, I would check out a Dodge Cummins.

    I agree that in my case the speedo and odometer are probably not accurate due to the larger wheels but. hand calculating nets the same piss poor 10-11MPG. Larger tires should NOT make that much difference in MPG vs stock tires! With the Duramax I should be getting at least 13-15 if not higher considering some on this forum claim 17-20, which sorry, I don't believe!

    I did find cool product that I would like to try. It's a solenoid operated exhaust bypass. I think the gas mileage issue in these Duramax's is partially related to all the smog crap they put on in the exhaust. Check out http://www.streetpro.us/PowerMasterPics.html
    I really don't know. I think if your tires are 10% larger than stock diameter, that affects the circumference greatly (10%) and hand calculating cannot make up for that. Let alone the electronic thinking the ECM and allison are doing for you.

    For instance, the DIC on my 6.0 1500HD 4x4 reads "average" 10 to 11mpg if I let it figure over 3-4 tankfuls of normal home and business driving.. My tires are 305/70/16 Wranglers and I guess they are about 10% larger than the toy tires which came stock on the HD.

    I cannot speak for the innacuracies of the DIC regarding gallons used because I doubt any of us know for sure we get an exact gallon at EVERY pump or not, so that's another variable.

    I do agree that I expect more than 10-12 mpg or I'll just keep my 6.0HD.........

  4. #64

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    2008 2500 HD crew cab, long box bought in March, don't drive it much yet, but with about 5,000 miles, getting between 17.54 to 18.81 hand calculated. Mixed driving.

  5. #65
    Join Date
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    Default 2008 2500 HD 4x4

    Just took delivery. 1 tank so far. Dash shows 14 - 16 in city. Highway driving can be good or bad; Seems the truck likes to run about 68 - 70 mph and 1800 rpm's. Running here the dash shows 22.6 mpg. Push the speed above 70 and 2100 - 2200 rpms and the mpg drops to 17 - 18 mpg.

    I read about the priogrammers and the results don't seem to justify the cost.

    I here (rumor) that the 2008's have a tattle tale in the on board computer. Do theses things void the warranty?

  6. #66
    Join Date
    Jun 2008
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    Well I bought the 08 Crew Cab short bed 4x4 chivvy with Duramax

    Riding home the DIC calculated 22mpg with 1hr of freeway only driving... round town averaging with that freeway run over the next couple of days, the DIC is calculating 14 to 15mpg... Dunno bout the accuracy til I do some hand calculating to check GPS speed to the truck speedo and then hand calculate MPG...

    I need to study this site more to find out about all the programmers etc because I do intend to run 305 70-17's on this truck

    By the way, what is with the cooler thingy for the fuel being down near the ground in front of the fuel tank with no protective skid plate?

  7. #67
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    There is a tattle tail feature on the LMM's that keeps track of the programming/flashes. This was relayed to me by the shop foreman and friend at the local GMC dealer.
    I recently did a turbo back exhaust and installed a "Spare" computer that was programmed mild upgrade/tow.
    Hand calculated my mileage in town is up 1-2MPG on the highway 5-7 MPG increase.

  8. #68
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    Quote Originally Posted by Sivrug2 View Post
    There is a tattle tail feature on the LMM's that keeps track of the programming/flashes. This was relayed to me by the shop foreman and friend at the local GMC dealer.
    I recently did a turbo back exhaust and installed a "Spare" computer that was programmed mild upgrade/tow.
    Hand calculated my mileage in town is up 1-2MPG on the highway 5-7 MPG increase.
    will the tattle tale feature void my warranty if I install one of these programmers ?

  9. #69
    Join Date
    Nov 2002
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    Littleton, Colorado
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    Default Mileage

    New 2008 GMC 2500 DC Crew cab....Colorado

    First tank....mt, mixed town and highway.....16.2 mpg

    Second tank.....mt... up the mountains to go fishing...mostly highway and 70 mph.....17.4 mpg

    Hand calc'd

    Slim
    2008 DMAX, 4wd, LB, CC
    SRW, 265/70/17

  10. #70
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    The ECM is supposed to keep track of the last 10 program reflashes.
    1998 Chevy k-2500 454 147,000 miles and climbing!

    1993 Pontiac G/A Big Block 3.3 Liter 140,000 miles and still going too!

  11. #71

    Default On Board Computer/Hand Calculation

    Here is a few pros and cons in the way mileage is calculated
    When you calculate by hand you have no control over the temp factors,wind,grade to a given point which all effect the rolling stance of the truck. l recently did a calculation using a GPS and the on Board readings. I went 25miles constant speed 55mph pulling a 10,000lbs load the GPS and on Board Computer matched up exactly in distance with an average of 12.5mpg on the Computer. The next day which included some tempature change (5degrees higher) and wind(almost at zero) did the same trip. I got 13.5mpg. The on board PCM calculates the mpg including the wind/grade of road you are traviling on and temp change where calculated by hand you do not include those factors. I also took out a 2008 5.3 l was tuning at my shop drafted a transport on the HWY at 55mph l was amazed watching the onboard compter change from 22mpg not drafting to 25mpg when l drafted the the transport l think the onboard computer are a little moe acurate than we think. It is no secret that the New Exhaust Filter System hiders gas miliage and the amount you can strech the Custom Tune without a Delet of the System. Just my two pennys worth
    Last edited by DmaxMaverick; 04-26-2009 at 20:42. Reason: Email to follow.

  12. #72

    Question 2009 Fuel mileage experiences

    I own a 2006 Chevy 2500 HD Dduramax w/ 31,000 miles. I'm looking at buying the 2009 GMC 2500 HD. I'm not too comfortable buying the new deisels with all of the emmission mandates from the EPA thus having a reduced Fuel mileage than what I already have. I'm just getting 17 MPG in combined City and HWY driving mostly HWY, (54mi HWY, 13mi City).

    What has been the AVG MPG in the new trucks now?

  13. #73
    Join Date
    Oct 2008
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    Cheyenne WY
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    Quote Originally Posted by agmakar View Post
    I own a 2006 Chevy 2500 HD Dduramax w/ 31,000 miles. I'm looking at buying the 2009 GMC 2500 HD. I'm not too comfortable buying the new deisels with all of the emmission mandates from the EPA thus having a reduced Fuel mileage than what I already have. I'm just getting 17 MPG in combined City and HWY driving mostly HWY, (54mi HWY, 13mi City).

    What has been the AVG MPG in the new trucks now?
    I have a 2009 GMC Sierra, Crew Cab, 4WD, with just over 3100 miles on it. I haven't had an opportunity to check highway mileage yet, but in-town I am getting just over 14 MPG. As with the other Duramax trucks I've owned, I'm confident the MPG will increase as more miles are put on the truck.
    2009 GMC Sierra, 2500HD, Crew Cab, 4WD, D-MAX
    2009 Chevy Malibu LT, V6
    2005 Jayco 323RKS, 3600W Onan Generator

  14. #74

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    Quote Originally Posted by agmakar View Post
    I own a 2006 Chevy 2500 HD Dduramax w/ 31,000 miles. I'm looking at buying the 2009 GMC 2500 HD. I'm not too comfortable buying the new deisels with all of the emmission mandates from the EPA thus having a reduced Fuel mileage than what I already have. I'm just getting 17 MPG in combined City and HWY driving mostly HWY, (54mi HWY, 13mi City).

    What has been the AVG MPG in the new trucks now?
    Unless you have a very compelling reason to get a 2009 I'd keep your 2006. The new trucks are nice with the exception of the gas mileage. I had a 2002 that got 14-15 City and 15-19 Hwy. My 2008 gets 10-11 City and 14-16 Hwy.

    The other thing to consider is that aftermarket chips/computers are not recommended as they will effect the new emissions equipment.

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  15. #75

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    There are no compelling reason other than getting a newer model with the added options and having a newer model before I retire. I like to research performance by asking others on thier experiences to get a better guage on what is going on with the new Truck inventories because the Manufactures and dealers will not tell the truth. I know fuel mileage ALWAYS takes a hit when new emmissions equipment are added I just did not know how much of a hit the new engines are taking. For me, fuel economy is first on my list when making truck purchase decisions.

    Sometimes I think it's done on purpose to kill the deisel engine for other than commercial use.

  16. #76

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    mpg for city driving will be all over the place.. I can put you on the loop in DC at 4PM and I guarantee 5mpg at 12mph average...
    And a similar story for highway mileage depending on how you drive..
    To give some perspective I have a 2007 Silverado 1500 4X4 with the small gas V8... I drive like a little old man
    (Wait! I am a little old man, ah jeez, and now we've changed Centurys... Damn I hate it when that happens...)..
    I get 16.8 mpg average in mostly rural driving with a bit of village tossed in (1 stop light) and once a week to the big city... You Duramax fellas with your 16 mpg average are doing just fine...
    Before that I had a 2004 Silverado 4X4 with the 350 V8 and got - TADAAAA - 16.5 mpg average...
    I just bought a new, dealer left over, 2008 Duramax 2500HD CC SB 4X4 and will be pleased if I get 16...

    denny-o

  17. #77
    Join Date
    Oct 2008
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    Cheyenne WY
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    I just returned from a round trip from Cheyenne WY to Ogden UT. I put a total of 997 miles on the truck, and was towing a 5th wheel for a combined weight of 17,970 lbs. I had the transmission in the Tow/Haul mode, and the cruise control set at 75 MPH. Given the fact there are only 7900 miles on the truck, the number of construction zones, and the number of significant grades on I-80 between Cheyenne and Ogden, I am not disappointed with the 8.8 MPG. The ole Duramax just kept on trucking.
    2009 GMC Sierra, 2500HD, Crew Cab, 4WD, D-MAX
    2009 Chevy Malibu LT, V6
    2005 Jayco 323RKS, 3600W Onan Generator

  18. #78
    Join Date
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    Hi,

    We're in the middle of a trip from Illinois to Pennsylvania and back, towing a 12k fiver.

    Hand calculated towing mileage at 55-60 mph (rolling hills, mix of interstate and secondary roads) has been right at 11.4 mpg. Hand calculated mileage tooling around without the trailer (driving between quilt shops and other attractions) for several days was 18.6.

    Puzzle -- I have yet to be aware of this truck regenerating -- even when I'm home and driving locally in suburban Chicago for months at a time.

    FWIW.
    Rich Phillips
    Member #27
    2019 K-2500 Crew Cab Z71
    Cedar Creek Silverback 33RL Fifth Wheel
    In The Past: '82 6.2 Jimmy Blazer, '93 6.5 GMC K-2500, '01 DMAX K-2500, '09 DMAX K-2500

  19. #79

    Default We'll see

    I just took delivery of my '09 Sierra 1-ton xtended cab on Monday. It's still in the break-in period, but mileage is hovering between 18 and 19 at speeds primarily in the 55 to 70 mph range. Average altitude around 5,000 feet here. Heading up and over the Big Horns today... steep grades up to just over 9,000 feet.. will be interesting to see what the average drops to.

    Thanks GM and UAW... You make one helluva truck.

    My son is getting 22 mpg with his 2004 3/4-T Duramax, but I doubt I'll be able to wring that kind of result out.

  20. #80

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    I have an 08 CC 3500 Dually 4x4 Duramax/Allison and get 14 driving and 10 towing a 24ft gooseneck. Truck has 10 thousand miles on it. Was hoping for more. Had a 03 2500 LB 4x4 with 245's that got 18.5 driving 80-85 mph. Really would like to see more mpg. The Dually has 17's on it but can't go to bigger tires because the duals would rub.

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