Our 2006 F250 6.0 diesel (stock, no tuner) gets 13mpg pulling one Jeep on the small trailer (5500lb'ish) and we got 10.5mpg with both Jeeps on the gooseneck (roughly 12,500lb) traveling to Moab and back. I typically tow at 65mph, slower if the speed limit requires it. Unloaded, the truck has...