I have a 4x4 5.4 01 Screw, Off Road pkg with 3.55 gears and 265/70r17's. My normal highway mpg is 17ish, but on a trip to OH and back I got 20 both ways, but I was on a turnpike most of it so it was very flat. Going to NM last year I had a high of 19.6 and a low of 15.5.
I usually get around 13-14 with the city/some highway mix.