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Keep RPM's at about 1800, this usually means driving slower.
Install a bed Cap or Tonneau cover
Install a smaller gear in the rear end.
Keep it tuned up.
replace heavy foot with a lighter one
12-20 hwy is the norm for these trucks, 4x2 getting the better milage of course.
Thats what mine gets on average. I've had several combos (5.0 5.0 F150 -350 and bronco) of these trucks . The only time I've ever got better with and auto trans is on a trip out west 17 Mpg .With standard I got almost 20 Mpg with cruise set out west. If I stay out of town I can add about 1 MPG . Good luck
At least take comfort that you are "in the ball park" at 15MPG. Keep your truck tuned up to the best that you can afford, swap out the O2 sensor if it has a lot of miles on it - they don't last forever, and tread lightly on the loud pedal. Flying a brick through the air has never been a fuel efficient effort!