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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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  #41  
Old 12-19-2007, 05:32 PM
RaiderCoach RaiderCoach is offline
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Default Update and test...

I have broken the 10,000 mile mark with my 07 LMM. I have not noticed any increase in mileage yet. I average 17mpg hwy and, sadly, about 10.5 in town. This is with very light footed driving also. The 10.5 in town is with the computer set to the factory settings (Bully Dog progammer). I just filled up with my first tank of B50, I'll use B99 when the weather warms, and reset the programmer to the next power setting. Speaking with the service manager at my Chevy dealership, they have had problems with the programmers causing the dpf's to clog, so I have left mine at the stock setting. With my '03 Duramax, I ran B99 often and found that there was much less soot produced with that fuel, so I'm testing to see if the combination of the programmer and the Bio-Diesel will result in improved economy. I will report back after a month or running this scenario.

On a separate note, I find that my computer is generally off by one gallon at fill ups compared to the amount of fuel that it takes to fill the tank. I attribute this to the remote start that I use every morning to take the ice off. I don't think the computer monitors the fuel usage while in the remote start mode.
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  #42  
Old 12-21-2007, 04:04 PM
kratos1 kratos1 is offline
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Default Fuel mileage

I am trading in my 04 LBZ for an 08 LMM. Any word on programmers available for the new motor. I have 138000 on my 04 and get 17-21 mpg depending on how I drive. I installed the Hypertech programmer and ran it on stage 2 the whole time which dramatically increased power and about 2 mpg. I was talking to a guy that bought an LMM about 6 months ago and said he's getting 15-19 depending on how heavy his foot is. A friend of his bought an LMM a few months later and is gettig 13-15 mpg. Difference being he put larger tires, 305 instead of 265 and lost about 4 mpg. I guess the key is leave everything stock. Neither guy has a programmer so the trucks are bone stock. I plan on leaving everything stock on mine with the exception of the Banks programmer, larger exhaust and cold air intake. If any one has any suggestions I'll listen
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  #43  
Old 12-31-2007, 09:21 PM
DaveRuane DaveRuane is offline
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Default

Quote:
Originally Posted by kratos1 View Post
A friend of his bought an LMM a few months later and is gettig 13-15 mpg. Difference being he put larger tires, 305 instead of 265 and lost about 4 mpg. I guess the key is leave everything stock. Neither guy has a programmer so the trucks are bone stock.
Without a programmer, the computer cannot provide accurate miles/gallon nor speedo, nor odometer. Going from 265 to 305 should be a significant error on all 3 of these measures. I bet he actually gained mileage if he only "sees" a loss of 4mpg. Does anyone have the math on the %difference from 265 to 305? I know that going from 245 to 265 causes a 3.9% error...
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  #44  
Old 01-01-2008, 07:32 PM
dustoff_doc dustoff_doc is offline
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Default Mpg

I bought an 07 3500HD in November. I have 4K and DIC is 15.2 MPG. I wish I had the MPG my 04 2500HD had a trade which was over 17 MPG all the time. Oh well! I still like the power of the new 07 just wish the MPG was better!
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  #45  
Old 01-06-2008, 06:50 PM
MrBSA MrBSA is offline
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Default Fuel mileage

I was going to post a question about my deteriorating fuel mileage but decided to jump in here and see if anyone can help. I have a 2002 2500HD Dmax (not sure what model engine that makes it) that I purchased used here in Canada with 130,000 Km. When I first got the truck I was consistantly getting around 23 to 24 MPG (Imperial) around town (some hiway, some town) and while towing the 5th wheel would get anywhere from 15 to 18 MPG depending on terrain etc. I was quite happy with those figures! I now have 144,000 Kms ot the clock and lately my around town MPG have been dropping steadily, first to about 21 and this week to 18 MPG. I convert my mileage with a calculator using the Kms travelled since previous fill up and # of litres to fill the tank. I don't think my driving has changed (not aggressive on the throttle), the truck seems to still run well, doesn't smoke that I can tell, so what's up! Any ideas what is causing the drop? As always I appreciate the comments and help.
Cheers,
Bruce
Vancouver, BC
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  #46  
Old 01-06-2008, 10:16 PM
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Stlheadake Stlheadake is offline
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Default losing fuel mileage

Maybe winter blend fuel? I notice that in the cold I do get worse fuel mileage. Are you using a winterfront to keep the heat up as well. I notice an increase in mileage when I use my winterfront in cold weather. The truck seems to run about 20 degrees or more warmer. I get plenty of heat, but the performance seems to be better.
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  #47  
Old 01-07-2008, 11:19 AM
MrBSA MrBSA is offline
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Default Mpg

Hi Scott,
yes I wondered about the cold weather aspect. I don't use a "winter front" ( I assume you mean something that blocks off cold air to the radiator). Maybe I should try that. It has been a little cold here at times and the truck does take a long time to warm up. I do occcasionally add a fuel additive but haven't been able to determine if it makes any difference. I would like to try bio fuel but can't seem to find a station that sells it anywhere around here. Also having trouble finding a 15/40 synthetic motor oil other than "Royal Purple" which is a staggering 7+ dollars a litre. When our trucks take 9 1/2 litres that makes for an expensive oil change!!
Cheers,
Bruce
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  #48  
Old 01-13-2008, 09:43 PM
RaiderCoach RaiderCoach is offline
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Default followup on Bio Fuel economy

OK, I ran two tanks of B50 (50% bio and 50% petro) in my LMM with the Bully Dog programmer set to the 'Towing / Economy' setting. Truck ran well, smelt horrible, and got worse economy than running at the stock settings with 100% petro fuel.

Even still, I'd like continue to run bio fuel...seems like the right thing to do. I don't know why the smell from the exhaust is so rancid though, on my previous '03 Duramax, it smelt like french fries or something, but on this truck with the advanced emission controls, it has a very acrid, harmful smell. Anybody have any insight to this? I'd hate to be running a fuel that's going to cause me to get cancer or something!

I did notice that speed has a very large part on the fuel economy on this truck. Unlike my '03, that seemed to get better mileage as you increased from 55 up to 75 mph, this truck goes the other way. At 55 mph I peaked with my trailer at about 13.5 mpg and at 65 I was at 8.5 mpg. Didn't even try 75 as I may not have had enough fuel capacity to make it to the next filling station!
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  #49  
Old 05-14-2008, 03:32 PM
Canuck76 Canuck76 is offline
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Default LMM Fuel Economy

I have a 2007 LMM on a 2500HD that doubles as a daily driver (no load). I have yet to get less than 15 MPG combined (DIC) roughly half city half highway after 18500 miles. Seems like the magic highway speed is 55 to 60 mph. I have scored as high as 23 MPG pegged on 60 mph (DIC).
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  #50  
Old 05-15-2008, 06:45 PM
Sivrug2 Sivrug2 is offline
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Default Bio in LLM

Raider,
I would not exceed 20% bio in the LLM. When the LLM goes into regen the bio will cause problems. First the regen cycle injects fuel later in the power cycle. First, Not all the bio is burnt and will cause dilution of the crank case oil. I run oil tests randomlly on all my diesels and after running 25% bio the alalysis came back way over. I do not have the research link infront of me but if you want i will find it. This breaks down the engine oil. Second, high ratios of bio can cause to the particulate filter to become clogged.

As for my mileage on the LLM, @18k I am averaging 17.2 combined. I am running 25% GTL with ULSD.
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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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  #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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Arrow

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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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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  #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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  #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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