2004.5+ LLY Duramax Fuel Economy Poll

[dannomatt]

Thanks. I'll continue researching. It seems that gauges are are universally recommended to go along with most modifications. Understood.
It's frustrating how many opinions there are regarding the re-programming of a vehicle's ECM.
The dealer who re-flashed mine insists that all they did was correct for tire size. (They also helped me determine that the tires were the main culprit in the loss of MPG)
The local diesel mechanic says the dealer unavoidably gave me some EPA / AQMD 'updates' that are probably causing additional fuel consumption. (Dealer insists that only speedo 'files' in ECM were opened)
A GM rep told me that the only ECM re-flash they support is not related to tire circumference. He even went as far as saying "...there are some parts of your ECM that are better left un-touched."
The 2-4 MPG loss isn't killing me, especially considering that I know the tires are most of it. My goal now is to determine if there are mods that can some of it back.
Great truck otherwise!
Dan

[DmaxMaverick]

Some mods can help improve your mileage. Problem is, at what cost? The reality is you may not see an ROI (Return On Investment) until 100-200K miles or more, or never (depends on how much you spend, the mileage increase, and how long/far you keep/drive the vehicle). Driving habit adjustments are free. However, in every case, YMMV.

[Diesel 4me]

Wishing that mileage could be improve don my Dmax as well. I have a 2005 with aprox 22,000 miles - stock sized tires and my DIC records aprox 13.8 while driving down the highway at 60 (Ca).... Engine stock for now ... no programmer

[JohnC]

Don't believe the DIC unless you have confirmed its accuracy. Calculate it based on mileage and fuel purchases. (Verify odometer accuracy, too....)

One easy way to check the accuracy of the DIC is to zero the fuel used when you fill up and see if it agrees with your next fill. Repeat a few times to get a feel for its accuracy.