Im getting about 15MPG empty, only have just over 7,000 miles, havent done any "heavy" towing yet, but dragging a few empty GN trailers averaged around 11-12MPG

 

The mileage really depends on the gears, I have a 2012 F450 DRW with 4.88 gears, and 17inch wheels 31 inches tall...I put on some 19.5s @ 33 inches tall, made quite a difference in fuel mileage. I normally run between 65 and 70 with some pretty heavy loads, my weekly MPG runs around 10-11 don't go by whats on the dash.