My 1999 F250 Super Duty 7.3 liter diesel, auto transmission, extended cab with 8' box, 4x4, and 400 lbs shell gets 14 mpg towing my 22' at near sea level and slightly less at higher elevations. My rig w/o boat weighs 8,100 pounds with full fuel load, one dog, and me. My speed is 55-60 mph...