[jocko]

the 8.1s were paired with Allison 5-speeds and 3.73 gearing, as opposed to 4.10 in the 6.0 trucks. The best mpg I've seen in my 01 8.1 is 13 - that was on a long highway drive with the cruise control on for hours at a time. In general day to day use (which is still mostly country road driving), I average around 10-11 mpg, just like Gary said. It's not likely your truck has rear gears as highway friendly as 3.73s (relatively speaking...) - I agree with Steve, the best you could hope for is a 4.10 in that thing, but it's not very likely, so your mpg would be notably worse than what is seen in the GMT800 series with the 8.1 drivetrain, generally speaking. At the very least, before you go down this road, you need to determine what your current rear end ratio is (unless you do swap the rear from the 02). If your 02 is a decent, driveable truck, I sure wouldn't yank the drivetrain out - the early GMT 800s with the 496 are desirable trucks - if you don't care for it, sell it for a profit and buy an overdrive unit for your 72. It is a large undertaking and not an easy swap.