My 2003 FX4 originally had a 4.0L, manual, 4.10s and 30.5" diameter tires. RPM at 70 was 2600 in fifth and highway fuel economy was 19~19.5 MPG at that speed, city economy lower.
I don't know if you are buying new or used but later FX4's run 255/70/16's which are only slightly shorter at 30.1". If your truck is new, it will take a few thousand miles for it to loosen up. After break in, I would expect your FE to be very close to what mine was.