I've never owned the 5 liter Ford, but that mileage sounds pretty miserable
to me. A lot depends on your particular driving habits...Have you ever tried
running at 60 mph when you test your mileage?

My '03 with the 4.2L V-6 only turns in about 14-16 in the city
depending on how I drive or with how much payload. On the highway here
in CA you must do at least 70 in order to not be killed and at that
speed I'm getting about 18-20

I have a '95 Bronco -- essentially an F150 station wagon -- with the 5.8L
motor, and I get about the same mileage as you. My brother also has a '95
Bronco with the 5.0L engine, and he also gets the same.

I get about 19-20 mpg on highway with cruise on 59
in a 2005/ F150  with the 5.4 Triton V8.   Running
K/N air filter and Tufoil mixed with the Ford Synthetic
blend Motor Oil.

I think the Tufoil is a major help as I ran it in my
Chevy with the 5 liter V8 and put over 240 thousand
on that engine.  Compression was strong and even on
all 8 cylinders when I got rid of that truck.

Don't forget to keep you're tire pressure right too
for improved mileage.