Help with gas mileage

[larryh1]

What am I doing wrong? Reading on the board I see people getting 17 to 20 mpg on their Duramax. I have 21,000 on mine now. I have installed a Superchip programer that I run on the safe tow mode. Performance Tow and Performane Mode did not help mpg at all. I am running stock exhaust, with a K&N air filter. I soft foot the truck trying to get good gas mileage. I am running 18in tires, but nothing wide or heavy. I would love to get 17mpg!
I generally stay around 14.9 - 15.5 @ 70mph. Running stock I get 14mpg. When towing a 7000 lb, 30' toy hauler I get 12mpg @ 70mph. I have tried 65mph and it doesn't increase that much. Maybe half a mile per gallon. Other Duramx owners have told me to get the Bulldog programer. Can anyone tell me if it will make a difference or am I expecting too much? Will the exhaust make that much of a difference? Oh, and I run my tire pressure at the max. setting. On the rare occassiions that I have put my foot in the truck it runs great!!! I do see a lot of black smoke, but only when I put my foot in it. Any suggestions.

Thanks everyone. This board on the people on it are a pleasure to come to.

Larry

[Kennedy]

This is one of the reasons that I do not promise MPG gains. Results can vary, but MOST will see a nice gain when I do a custom tune.

[Mark Rinker]

What is the diameter of tires you are running? Have you adjusted your ECM with the non-stock diameter?

It would be interesting to swap back on stock rubber, keeping as many other variables the same (tuner, driving style, etc.), and see what happens with your mileage...

Braking habits also play a large part in fuel economy. I have learned to drive my trucks/trailers so I only brake in the last 30-40 feet before coming to a stop. That means ALOT of rolling deceleration. Most of my speed is scrubbed off by tow/haul downshifts, and in the case of the Kodiak, the engine brake - while the engine is basically idling. Its not a fast way to drive from point A to point B, but saves $$$ and brake parts!!!

[larryh1]

The tire size is LT275/65R. Diameter is 32 inches. Yes, I changed the ECM with the programer. The problem I had with the stock rubber was it was such a light tire. I am involved in motocross racing through my children and am off road pulling the trailer. I wanted something more stable and able to go off road. I don't go "4 wheeling" in it. Its just that it never fails, when we are at an important race.......its going to rain, and it is nice to be able to drive out. I didn't see a drop in mileage when I went to the different tires, but the truck was still new and not getting good mileage anyway, 12 / 13mpg. I agree with the braking habits. I'll pay more attention to that. I can feel my tow/haul braking also. I am always amazed how that works and how good it works. With the price of fuel I will do anything to save some dollars. We are cutting back on our trips simply because the price of fuel is cutting into the budget too much. What about the exhaust. Would it pay to go to a larger pipe and do away with the cat?

Mr. Kennedy, I have been to your web site and have read your responses on this site. I have developed a respect for your opinion and wish I was close enough for you to do the work on my truck. That goes for you too Mr. Rinker.

Thanks for the response. I'll keep trying different things.

(this board and the people on it are a pleasure to come to)

Larry

[Mark Rinker]

Reducing backpressure with a free-flowing exhaust should help. Same on the intake side. Biggest factors are still driving habits - speed, braking I am convinced, and neither of those have an initial cost to recoup.

[Kennedy]

The tire size is LT275/65R.
Mr. Kennedy, I have been to your web site and have read your responses on this site. I have developed a respect for your opinion and wish I was close enough for you to do the work on my truck. That goes for you too Mr. Rinker.

Thanks for the response. I'll keep trying different things.

(this board and the people on it are a pleasure to come to)

Larry

Mail order tuning works for many. In fact most of my customers are from out of state.

[appleman]

I`m new on here and I am looking at a new 08 Duramax. I`ve talked to a few guys about their milage and they are talking 13 mpg stock. with 3.73 rears. Since I`m new to the diesel I don`t which motors they are but I see on here there is a LLY LLM on some of the reports. I was told that the 07 and 08 that you could not not program these trucks for economy because of emissions . Always been Chevy man but That Cummins is nice and according to the dealer it gets in the mid teens in the city. Just wishing the Chevy will do the same. Any help here would certainly be appreciated .

[EdHale]

We all have discussed this many times, but I'm convinced driving style is the major reason some do and some don't get good mileage with the Duramax or with any vehicle for that matter. I have friends that don't get good mileage and others that do. I am not an aggressive driver and I always use the tow-haul for grade braking. I coast into a red light or stop sign rather than drive up to them at full speed and slam on the brakes, etc. I don't act like I'm on a drag strip when leaving the stop either. I get 20 mpg more or less with my 2006 LBZ when driving 65-70 mph. If I speed up to 75-78 I get 19 or slightly less. I live in Georgia so most of my driving is relatively flat. These numbers are hand calculated because my DIC is always a little lower. No problems with mileage for me. I would suggest that those that don't get good mileage can't help but enjoy the tremendous performance capability of these trucks a little too frequently. It is tempting as we all know.

Ed

[Barkbuster]

I must drive a lot like Ed as I get similar mileage to him. I also have a ScangaugeII. One of my favorite gauges on it is mpg gauge. It is real time and very surprising how often the truck is getting 30 plus mpg,or more. It also gets less than 5mpg just getting her rolling.I think the Scangauge has helped me get better at getting good mileage most of the time.My trucks electronics are still stock but would like to try a tune or tuner,and or an exhaust, but not till next year.

[NutNbutGMC]

I`m new on here and I am looking at a new 08 Duramax. I`ve talked to a few guys about their milage and they are talking 13 mpg stock. with 3.73 rears. Since I`m new to the diesel I don`t which motors they are but I see on here there is a LLY LLM on some of the reports. I was told that the 07 and 08 that you could not not program these trucks for economy because of emissions . Always been Chevy man but That Cummins is nice and according to the dealer it gets in the mid teens in the city. Just wishing the Chevy will do the same. Any help here would certainly be appreciated .

The 08 will have the LMM engine. It has several additional components of environmental friendliness that the predecessor LBZ does not have. Thirteen MPG sounds about right but that is relative to a lot of variables. There's a thread here about mileage results for the LMM ==> http://www.thedieselpageforums.com/tdpforum/showthread.php?t=27168

Kennedy Diesel (www) has current info on his tunes, but if I recall correctly, he is sitting in the back row awaiting some additional R&D for the LMM, but please don't quote me on that.

The 08 LMM comes stock with HP and torque to spare for all practical uses (IMHO).

Again in my opinion, the Cummins is a fine engine. I wish it was in the GM line, but it's not. Don't listen to a dealer. How the heck do they know how you drive and what expectations that lie deep within you for dark desires of performance? :D

You'll get what you get, but from what I have read, the Cummins will be as reliable and mileage-friendly as will the Duramax (all stock provisions). I prefer the GMC quality of provisions and resell.

I have no expectations of MPG excitement. I do not even go there with a light-duty diesel engine. If it's not 30+ MPG as is the passenger automobile I am not impressed by anyone's opinion. It's not a sweat to me to wrestle with 1 - 4 (more or less) MPG when dealing with these engines. I use the truck, pay the price of fuel, drive it like I need to, and the rest is insignificant as to economy. Mileage and economy is limited within the genre of light-duty diesel engines. In my opinion, it's just the way it is. Mid-teens MPG is all too common and documented time and time again.

Fact is, from here on out, you'll have to purchase a diesel machine with the enviro-friendly components on all diesels.

Sooner than later, someone will provide a work around for additional performance in one way or another.

I'm just an ordinary driver and forum browser, so someone else may have mileage that varies...I've never seen 30 MPG from any diesel, but everyone has different results.

[6.6Turbo]

I have a cold air intake and 5" exhaust with edge chip and cruising on the highway I think I am getting worse mpg's than bone stock. I think mph and wind play the biggest factor. 55-60 I am two mpg's better than 70-80.