Anywhere from 9.5 to 11.5. Seems the wind plays a big role. I have been running 5startunes (93 & 87 performance) and stock still gets better fuel mileage consistantly. This is trended over a minimum of 5 tanks each tune. 93 performance is the worst but not that noticeable with ave. mpg at 10.1 and stock getting ave of 10.8.
1999 F350 4x4 5.4 gas, 5 star tunes, SPD Y-Pipe, 4.56 gears, procomp trac bars, 35" A/T cooper's, 16x10 Ion -38offset wheels.
"My truck can get me out of trouble but I don't go looking for it."