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Duramax 6600 Welcome to the Internet's first Duramax 6600 diesel discussion forum for the LB7, LLY, LBZ, LMM, LML, LGH & L5P RPO code engines. Tips on performance, fuel economy, troubleshooting and more.

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  #51  
Old 05-16-2008, 12:59 PM
craigth craigth is offline
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Unhappy Consistent average MPG is still 10-11 with 6K miles!

My average mileage is still 10-11 with 6K miles on my '08 2500HD Crew 4WD. A buddy of mine has an '08 3500 same model with 20K and other LMM work trucks. ALL get the same low MPG! Not sure how you guys are getting the mileage you claim!! How are you calculating your MPG?!

I'm very UNHAPPY with the mileage and am considering selling/trading for a Dodge Cummins. There is no way an '08 that is six years newer than my '02 should get 5 MPG less!
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2008 GMC SLT 2500HD Crew Cab SWB 4WD ZLT
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Sliding Rear Window
Integrated Trailer Brake
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Nitto Terra Grappler 305/70/17 Tires
Spray-In Bedliner
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  #52  
Old 05-17-2008, 05:14 AM
Sivrug2 Sivrug2 is offline
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Craigth,
Calculating MPG is pretty simple. Never trust the on board info.

With your LMM your mods are hurting your mileage. The 6" lift and 305's are doing a 4-5 MPG hit. My experience with lift and big tires has always shown this. On my 2004.5 Duramax the pickup went from 19-21 MPG highway to 12-14 when I lifted and 305's.
I think the best mod for mileage is a Scanguage and using the "right foot mod".
The only time my mileage on the LMM is sub 10 is when pulling a 24 ft goosneck with a 18K backhoe on it. If I drive like a bat outta hell I can even get 4-5 MPH.
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  #53  
Old 05-17-2008, 12:53 PM
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More Power More Power is offline
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Those who shift their wants & needs more toward fuel economy should consider modifying their trucks along these lines:



Lowering the truck and adding aerodynamic improvements will help with fuel economy.

Jim
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  #54  
Old 05-19-2008, 10:09 AM
02bymorris 02bymorris is offline
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Join Date: Feb 2004
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Default Lmm Fuel Economy

my 08 Silverado LTZ crew-short box LMM averaged 17.5 local driving. I swapped the ECM to a Kennedy custom and piced up 2 more mpg. have not tested it yet on a trip. all stock except for the ECM
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  #55  
Old 05-19-2008, 10:31 AM
Runaway_Dmax Runaway_Dmax is offline
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Quote:
Originally Posted by 02bymorris View Post
my 08 Silverado LTZ crew-short box LMM averaged 17.5 local driving. I swapped the ECM to a Kennedy custom and piced up 2 more mpg. have not tested it yet on a trip. all stock except for the ECM
Even the exhaust system is still stock?
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08 Blue Granite Silverado LTZ, DMax/Ally, CC/LB/4x4, Cab lights, Camper mirrors, Brake controller, Sunroof, Park assist, and skinny tires

06 Jetta TDI

00 Jayco 263RK (2nd home)

upgraded to an "09" Ranger 619VS/full windsheilds, w/an ETEC 225HO, T8 kicker, Minnkota 101 Terrova, and Lowrance electronics.

Real Trucks have LONG BEDS!
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  #56  
Old 05-20-2008, 05:56 AM
50+ 50+ is offline
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Towing I get 10-11 and thats towing heavy, gross wt of 19k +. Local driving I get 14 -15 and the dic is usually around 15.4. On a highway run with few starts and stops and keeping the speed at 55 - 60 it will do 19+. On the same run at 65 it knocks mileage down to 17. All hand calculated. I can usually tell a regen if I watch the dic because it will go down about 2 mpg from where it normally will be. My LLY would do a little better than the LMM, but not much. I like the truck.
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08 2500HD LMM D/A Ext cab sb LT1 4x4 Greystone Metallic. 265 75r 16 Mastercraft Courser C/T 10 ply's. B&W GN. Pull 20' STL Featherlite Combo trlr.
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  #57  
Old 06-09-2008, 12:46 PM
RaiderCoach RaiderCoach is offline
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Default Another mileage update...

I now have 20K on my '07 LMM. My truck has 33" tires and a 4" lift, I'm sure that hurts mpg a bunch. I'm also running a Bullydog programmer at the medium (tow / econ) setting. Unloaded freeway driving I'm averaging 19 mpg and in town driving about 13. Loaded with trailer (about 12000 lbs plus truck), it drops down to under 14 on the hwy. I have found that the speedo correction from Bullydog does not work. I called them, but received the 'we haven't heard of that from anyone else so there's nothing were going to do about it' response. But, the speedo is off by about 3 mph, which would affect the mpg upward a bit, but I'm not a good enough mathametition (sp?) to figure it out. So the mileage is improving as the truck gets more miles on it. I would also agree that 55 - 60 is the sweet spot, at 70 I drop by about 2 mpg. Adversely, my '03 duramax improved it's mpg as the speed increased...
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  #58  
Old 06-23-2008, 04:12 PM
Ric Ric is offline
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Default New Here

Thinking of a new Dmax Chevrolet 2500HD 4x4

Reading all of your woes regarding mileage, how many of you who are hand calculating mileage, know that the odometer is reading correctly?

I've always thought that this plays into alot of folks differential in mileage claims...

And if all your odometers are accurate, is it possible that some here have different rear axle ratios causing different mileage as well?

MAN I don't want a ford.... ptt flaaaa yuk.... there I got it out of my mouth! But I did expect these newer diesels to be fairly efficient????
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  #59  
Old 06-24-2008, 09:05 AM
craigth craigth is offline
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Quote:
Originally Posted by Ric View Post
Thinking of a new Dmax Chevrolet 2500HD 4x4

Reading all of your woes regarding mileage, how many of you who are hand calculating mileage, know that the odometer is reading correctly?

I've always thought that this plays into alot of folks differential in mileage claims...

And if all your odometers are accurate, is it possible that some here have different rear axle ratios causing different mileage as well?

MAN I don't want a ford.... ptt flaaaa yuk.... there I got it out of my mouth! But I did expect these newer diesels to be fairly efficient????
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
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2008 GMC SLT 2500HD Crew Cab SWB 4WD ZLT
Black / Titanium
Sliding Rear Window
Integrated Trailer Brake
Billet Grill
Pro Comp 6" Lift Kit
Helo 17" Wheels
Nitto Terra Grappler 305/70/17 Tires
Spray-In Bedliner
Weather Guard Low Profile Aluminum Saddle Box
Remova-A-Ball Gooseneck Receiver
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  #60  
Old 06-24-2008, 10:59 AM
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Quote:
Originally Posted by craigth View Post
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!
What sort of fuel economy did you get with a completely stock truck, stock tires, stock programming, etc., and driving at a constant 65-mph on the Interstate?

Jim
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