I have a 04 burb w/ a 8.1 I get 8.5mpg in town . There arent many choices as far as mods but I like the 496. I had a 03 silverado 2500 w/ a 6.0 that got about 9mpg. If I could afford it I would put a duramax im my burb but since I dnt Im glad i have the 8.1. Ive hooked up to a 6.0 and pulled it sideways

 

I've got both as well. To modify get the 60 however both of mine are pre 08s so they have 4l8e. The 08 will have the 6 speed which will help out a lot. My cammed and supercharged 8.1 runs as good as the wifes 2011 denali granted there 3k lbs lighter