I have a 2016 3500 Express with a 6.0 liter, a 24" high top, and a Quigley 4x4 conversion. Overall I'm getting 14 MPG, 16 on the highway, and 12 around town. Before I got the high top and 4x4 I got 20MPG on a trip (trying hard to get the best mileage I could)