My 04 3500 single wheel quad cab 4x4 Cummins with 4.10 gears pulling my 32' LQ trailer got between 9 and 12 MPG depending on how hilly it was. Unloaded I could get up to 25 MPG going 55 MPH on flat terrain. Talking with people who have new 6.7 Cummins trucks get about the same MPG as the people I have talked to with 6.4 trucks, so if that is true then why pay the extra $8K for a Cummins? Diesel is cheaper than gas around the Chicago area, but even so it would take decades to pay for it compared to a gas truck.