I have Gladys, my 1975 F250HD Camper Special. 351M, Weiand Action Plus intake, Street Demon 625 three barrel, T18 4 speed, Dana60 with 3.73 gears. She routinely knocks down 15-17mpg and could probably do better if I didn't cruise her 60-65, or dump the humongoid secondary door open on a regular basis. She has 235/85/16 tires so they're pretty tall and that helps. I just bet were I to limit speed to just 55 and never crack that rear barrel, she'd knock on 20's door. That's impressive for a truck with an 8100 pound GVWR. Of course, I keep her in good tune, too.
....and that's with 1970s technology.