You need to tell us what rear end gear ratio you are running when you state your mileage. I had a 91 F350 dually w/ 7.3 diesel and a 4:10 ratio and now have a F450 chassis cab dually with the 6.0 diesel and a 4:33 ratio (both auto and crew cabs) and I was towing the same 5th wheel at 14,000...