I drive an 18-wheeler, a 1997 Freightliner Fld with an M11 Cummins and a super-10 transmission. I get as high as 8.5mpg ave. Bob-tail (no trailer) and between 5.5 and 7 mpg depending on total weight, Max of 80,000lbs on us highways with one 53' trailer. Hope this gives you a rough answer!