Go with a good computer tune and maybe a cat-back exhaust. Then just try to find the best driving style for good gas mileage(drive slower at highway speeds if you can). That 4L80E transmission and the weight is what really pulls it down.

 

I work with a piston engineer that used to work for Mahle Pistons. He helped in the development of the LS1 pistons. He told me that the calibrations for ¾ ton and up trucks are much richer the half tons to help keep the pistons cool when towing / hauling heavy loads. They can get away because they fall into a different emission category then there half ton siblings.

By the way, my 01 Sub 2500 2WD get 9.7 city / 14.5 highway. Has 75k on the clock and I replaced fuel filter at 69k.

I was hoping to find some on this forum that had a stock 6.0L recalibrated and found and increase in there mileage.

 

might try changing your O2's. i have seen as much as a 30% decrease in mileage due to old O2 sensors.