Like any turbo vehicle, you aren't burning a lot of fuel unless you're in the throttle. My friend's 7.3 liter Excursion 2wd gets 17 city, 22 highway on a chip with 160+hp over stock. Compare that to my Dodge Ram 4x4 with the Hemi that gets 10 city, 14 hwy.
Make a few WOT runs and that mileage drops. You can't change physics while you're shoving 7000+ lbs down the road at absurd velocity.