Mine is a '06 3500, Cummins, 4 X 4, auto. Not towing, I get almost 19 mpg as long as I keep the speed around 70, any higher and the mpg drops to approx 17. With a 26' enclosed tag trailer loaded to the max (10,500 lbs), I get about 12-13 mpg, unless I try to run over 70, then it drops to about 10.5-11. There is no lack of grunt here though.

Brian