I'll second the tire psi info. You should run 60-70psi.
If you haven't yet, check out
www.ls1tech.com/forums and scroll down to the Truck section.
Spend the money on the PCM (try
www.pcmforless.com,
www.nelsonperformance.com). You need to tell them that you're looking for MPG. I think one of the biggest improvements they can do is lockup the torque converter earlier, and remove/reduce GM's "torque management".
I wouldn't recommend intake/exhaust; that's for people looking for more "peak" power. The factory "cat-back" actually flows pretty good for a stock motor. But, if you're gonna do it, do it right:
www.dynatecheaders.com Long-tubes running through stock muffler is a good setup.
Something else to help reduce parasitic loss is converting to electric fans and maybe a smaller crank pulley.
And once you've seen it all, check out:
www.ststurbo.com
Good luck,
Michael
'01 Yukon XL 2500 w/8.1L and 3.73s
MPG: 10.5/13.5 bone stock w/115k miles
[This message has been edited by handfulz28 (edited February 01, 2005).]