When you get into the tall trailers, oftentimes the one ton trucks suffer fuel mileagewise, 6-7 MPG becomes common. We went on a 940 mile trip recently with our Freightliner 70 "Toter" pulling a loaded 36" gooseneck(11'6" tall) and averaged 11.3 MPG. The "Toters" just get better mileage pulling the tall trailers.
