if you change tire sizes and dont adjust the pcm to recalibrate for the
tires, will that make the DIC appear to get better mileage or worse, i
know that with the bigger tires, you are actually going faster than the
speedo says but ive heard both sides to the mileage appearing on the
DIC and am looking for an answer. my argument was that the dic
would appear lower because you are actually going a farther distance
and the dic doesnt know that, it thinks you are going less miles ?

It will appear to get less mileage. Bigger tires = less turns/mile = less miles recorded/gal.

I just bought a 2004 Duramax crewcab 4x4 and would like to know what I can and can't do to it. The first thing is what tires and wheels work the best for sand dunes, snow, rain, plus carry the load? I also would like to iprove the milage per gallon if possible. Please help, thanks, John.http://www.dieselplace.com/forum/smileys/smiley25.gif

You don't ask for much, do you? http://www.dieselplace.com/forum/smileys/smiley3.gif The tires that came on it are probably the best you will do for mileage, unless they have agressive tread. Mine came with Bridgestone R265, which is a highway rib. It seems from what I've read that bigger tires always hurt mileage, no matter what the tread. For sand, you will need huge flotation tires, for snow you need narrow tall tires, for load you need lots of plys. I think you need to build a tire rack in the bed and carry about 4 sets of wheels for the different conditions.http://www.dieselplace.com/forum/smileys/hihi.gif

http://dieselplace.com/forum/smileys/smiley4.gifThanks for the input. Is there a way to improve the fuel economy, the guy from the dealer-ship said not too much is out yet for the LLY, but in the past banks was the way to go. Also he was telling me not to go very much larger than a 285/75/R16 do to milage, he said this size will not effect milage. Edited by: 04DMCC4X4

Read the topic "disconnect EGR" in 2nd gen duramax lly, go to page 21. A guy named 'reliver' is making EGR block off plates. I bought one and installed it, but haven't tried it out yet. All purchasing info is there. Read the whole thread about mileage, etc. I put propane on mine, but if you can't get propane cheap, I don't think it will save you much.