I owned a 2001 chevy 3500 single rear wheel 12ft box truck with a 350 5.7 L motor and 4Le80 trans 4.10 rear gear it would average 8.5 mpg it did have the box I call it dock height so it did not need wheel tubs in it. I know own a 2007 freightliner mt45 step van with a cummins 5.9 and the allison...