I have a 96, F350, 7.3, ccdrw, 5 speed with 4:10's, stock and unloaded, freeway, cruise at 65 mph, the best I've ever got was 16.9 mpg
I also have an '02 Excursion 7.3, with 3:73's, also stock and on trips cruise set at 70 mph, it'll get 20 mpg all day long. Both are 2 wheel drive.
But if you...