fuel mileage w/ Duramax

[ct0218]

Bob
It is possible that since the 2008 (?) MY went to 265/75 tire sizes that it allowed that size to be programmed in prior MY GM trucks. I know that until recently the only size tires that could be programmed in was the 215/85, or the 245/75, which is almost exactly the same circumference. I haven't checked recently since they went to the slightly larger tire and wheel. Most guys went much larger size and that did not work. My neighbor has had several 2003-2005 GMC trucks and neither local dealer was able to change the tire size when he purchased them. The dealer installed larger tires/wheels but never could get the change to take. I remember reading many threads on www.thedieselplace.com over the years where the size was changed but as I remember only 1 or 2 aftermarket programmers, like EFI or Edge, would actually change the size in the computer. I haven't kept up with this lately since I have no intention of changing sizes, and I know that going any larger with my truck will affect my towing, but there should be a lot of threads on that forum addressing that size change problem. Like I said, if the larger size currently in use has allowed it to be retrofitted to prior model years, then that is good for those doing it.

 

[Bob&Patty]

Clark, what I know for a fact is that any GM truck can be programmed up to and including a 33" tire. I have not checked on the the new body style's yet, but will ask about them. Just to clear something up though, any module (computor) that is replaced in a GM car or truck "has to be programmed to that vehicle". It is done over GM's tech line using the OBD11 and CANDI MODULE (after 2006) scan tool hooked up to a computor in the tech's work area. Some of the newer ones, are hooked up the same way and also hooked to a phone line (PITA). This is a direct link to the GM data base. Confussed yet? There might be a possibility that the shops that people have gone to do not have someone that can do the reprogram, wont do it for free, or just wont do it. I hope you understand what I was saying in my previous post. Not picking on you or anyone else. Sometimes I think people are miss-informed. I dont know about you, but anymore I have got to the point that I dont even believe what I see with my own eyes. Just lots of bad information out there. Bob

 

[ct0218]

Doesn't that just reprogram the ECM and not the TCM? At least, that is what I understood. You work with it, I don't, but I was with my neighbor after dropping off his truck and that is what both dealers were telling him. There must have been 50 threads on this also, so few must be aware of it being possible. And that is entirely possible considering all the misinformation on the injector replacements.

 

[itsbeertime]

Thanks for the replies on reprogramming the computer after changing tires. I specifically asked my dealer BEFORE I did it if they could/would reprogram it and they said no problem. One tech swore up and down you couldn't do it (he's ALWAYS an old grump), but the tech who actually did the work *claims* he reprogrammed the computer for the new 265's. Now, there is no way for me to know if he *actually* did or not and they may have been telling me they did just to keep me happy. It would be nice to know for sure though. I guess I'll always be wondering.

 

[porthole]

Use a GPS.

Set the truck trip odometer, reset the GPS trip and compare mileage at 100 GPS miles.

as long as you have a relatively clear sky for the GPS antenna your mileage will be very accurate.

 

[itsbeertime]

Use a GPS.

Set the truck trip odometer, reset the GPS trip and compare mileage at 100 GPS miles.

as long as you have a relatively clear sky for the GPS antenna your mileage will be very accurate.

Thanks for that tip. I've been putting off getting another toy (read GPS), but there's another reason.

 

[Bob&Patty]

Clark, when I get a chance, I will make a copy of the procedures for the reprogramming. That should clear up any questions about what is done. Hey BEER, easier yet. Get on the interstate, set your cruise at 60 MPH, set your trip meter to zero, drive for 1 minute and check your readings. BOB

 

[itsbeertime]

Clark, when I get a chance, I will make a copy of the procedures for the reprogramming. That should clear up any questions about what is done. Hey BEER, easier yet. Get on the interstate, set your cruise at 60 MPH, set your trip meter to zero, drive for 1 minute and check your readings. BOB

@Bob: I'd love to see a copy of the procedures. That way I can walk in there and ask to be shown that they did it.

@Bob: 60 on the Interstate??? HAHAHA! I'd get run over and killed. Hmm, maybe I can find a side street though...

 

[NWTFHunter]

Mileage

I have a 2006 GMC 2500 Duramax/Allison and tow a Bighorn 3055. Had everything weighed upon arrival at the Rally last week. Truck & trailer = 20,175 lbs. I got my best mileage ever on this trip, 13.0-13.2. Most of the route was interstate and I drove 58-60 mph. I have a new set of Goodyear tires on the BH and sure if they helped or not. My milage prior to them was about 11.5-12.

 

[ct0218]

Bob, thanks--I would appreciate that. Some dealerships are just impossible to work with, and do everything they can not to help you. I got all my injectors replaced, no questions, no charge, in 1 day. The neighbor I spoke of had a GMC that was worse than mine, they replace 1 injector after 4 months of denying that it was defective, and charged him a deductible. It was in the shop for 10 days. He visited my NC home the following month, and got them all done at the dealership I used, for the same no-charge as I did. Nothing changed with the truck, it was bad injectors the whole time, yet the Tallahassee dealership gave him the runaround and overcharged him. They tried to charge him to replace the 7 that they wouldn't change under warranty, and stated that GM had denied the claim. The Asheville dealership said "no problem"
and replaced the 7. So, as you can see I believe a lot of things I have heard was impossible may actually be possible, you just have to show them in black and white. Some techs are poorly trained, some are just lazy.

 

[Bob&Patty]

Clark and Beer, your welcome. Yes Clark i know what you are saying. Maybe thats why some of the dealers are being closed down. Gives a bad image of GM. Not with standing, the dealer I work for will bend over backwards to help. As soon as I can get a copy of the procedure's, I will figure out how the post them on here. Fair enough? Bob

 

[ct0218]

Sounds good to me.

 

[Dmaxcc6spd]

We just did about 1300 miles from near Cincinnati, OH to Charleston, SC. We averaged 14.15 mpg for that trip. The mods are: K&N stock replacement filter, Kennedy Diesel ECM and 4" exhaust, 265 tires with ECM reprogrammed to match. I had trouble staying under 70mph. My truck likes to run 65-70 on the interstate. This trip also included I-40 and the Jelico mountain climb. I didn't downshift much on the way to SC, but did more of it on the trip back. Seems to like climbing hills without OD and I let the engine rpm sit at about 2500.

I couldn't be more pleased (well a bigger fuel tank would be nice!)

 

[R.Pratt]

i have the lmm 08 cc sb with a 3850 cyclone
driving from lakeview oh to enterprise al holding 70 i was pulling around 8.5
dropping to 60 got me 9.0 empty freeway i pull around 20 - 22
just have to watch out for the famous "regen" cycle

 

[PUG]

02 duramax boosted to 305 hp (+75) pulling 3950 at 18,500 lbs. Get 9 to 12 depending on hills.

 

[Chainsaw]

I *used* to get approx 11.5 according to the trip computer. I recently upgraded my OEM 245 tires to 265's and according to the trip computer I have lost 1.5 to 2 mpg. Not sure how that works, but it sure stinks.

did you have your computer changed when you changed the oem tires? It takes about 2 minutes to do. I had the same problem when I changed tires and went up.

 

[ChopperBill]

did you have your computer changed when you changed the oem tires? It takes about 2 minutes to do. I had the same problem when I changed tires and went up.

I had the dealer change the setting on the computer to 265's and it made no difference on the speedo. Found out later that all it does is make the ABS work in conjunction with the 265's. Installed the Edge w/Attitude and it will reprogram the speedo.