Sticking to the topic, with info on Chevy's!
My configuration is an 09 2500 HD 6.0 gasser, 6 speed, 3.73 gears, ext. cab, Z71. I pull a Wildwood 27 RBEC, I think it weighs in around 7500 to 8K. My truck very seldom pulls in 6th gear, and will only do it running 70 or so. At that speed if I'm lucky it will average 10 mpg. The 6.0 has absolutely no trouble pulling that trailer at any speed up any hill. In comparison, I started pulling the same trailer with and '03, 1500, 5 speed, 3:73 gears, ext. cab Z71 and same color. I could pull at 70 and get 12 mpg on a good day but however if I stepped it down to 65 it would get 13.
The kicker was driving the 2500 with a load is effortless. With the 1500 it is a full time job. Plus the tranny temp on the 1500 ran much warmer than the HD.
If I were to change today? Ford Eco tech V-6. Better mileage and prettier!! That coming from a Chevy guy??