My 2011 F350 gets around 14.4, but I have a 6.7. My 1996 F250 with a 7.3 gets 14.9. It has a cat-back exhaust, Edge tuner and AFE intake. I mostly drive it in the city. The 2011 is more highway & towing a 19' boat.
Of course if I didn't drive like I stole them, I'd do a little better!! LOL!