I have an 04 3500 with the HO 305/555 motor. I average 19 on short trips, empty, to 22 on highway, empty. See my sig for the details. I get the 22 in the KY mountains, but have to run 55mph, when I see that. I get about 19 running 70-75, and hold 18s running a lot higher. All these numbers are hand figured from fuel slips. My overhead is about 2 off to the low side.