My buddy has a I think a 02' F 350 Diesel, he swears it gets over 20 mpg. He has some sort of chip in it and a K&N filter in it. I'll see if I can find out more. I'm getting close to 21 with my new truck which is a 4.6. Rear gears imo have as much to do with mileagae as anything. My 05' had 4.10's, could rip a stump out of the ground but 14 mpg was the max. my new truck has 3.55's.
Charlie