My 1989 F250 2WD Supercab Long bed with the 460 CI fuel injected gas motor and non-overdrive automatic gets about 6 to 7 mpg in town and about 9 on the highway. And that is not pushing it for performance. The carburated version of a couple years earlier got about 1 MPG less.
At a very steady freeway speed with no off freeway driving it will get up to 10 MPG.
Water mist injection took it to 12 MPG overall from Portland to Tucson so you might want to try that.
BTW its designed usage was towing a 35' fifth wheel trailer thru the mountains of the western US without slowing down for those little hills in the Rockies.
It certainly didn't slow down coming up from Tucson to here packed to the top of the 2 foot high bed side rail extensions and towing a packed Jeep.