I have a 2500 HD crew cab with a 6.0 and 410 gears and at 65 mph the engine is turning 2300 rpm's. As far as gas mileage not towing at 65 mph I get 13.5 max and while towing our 33RLDS at 60-65 I get between 7 and 7.7 miles per gallon max.
In conclusion I do like the 2500 HD as is performs as expected and then some for towing.When towing a TT you get what you get for gas mileage however if I had to use it for a daily driver not towing l would say the gas mileage sucks and would get a small car to drive daily which I did or get a smaller TT that can be towed with a half ton and get that 17 mpg.
Chevrolet 2500 HD Crewcab
2013 Jay Flight 33 RLDS