On a roadtrip from DC to Birmingham, Alabama and back I averaged 21 mpg. My right foot is pretty damn heavy and I was doing 75+ mph for the majority of the trip (if traffic allowed). I have an 07 rcsb 4.8 with 3.23 gears, a 93 tune, and a cai. I feel that a set of gears and a good tune would probably help out, but like Pl4yboy stated, with that 60e it's just lugging alot. Throw a 6L80 in it It's got deep gears up front and two overdrives. I would get gears myself, but Im going to convert to the six speed as soon as funds permit.