I have an advantage in that I have a diesel, a manual trans (but no overdrive), and a standard cab.
My starting mileage is the same as your goal.
But with relatively minor changes I brought my 1983 F-250 from 15mpg up to 25mpg average with mixed loads up to 3 tons (28+ running empty)
Vehicle efficiency upgrades: 28.5mpg (so far) in 2.5ton commercial truck
Its easier to have big increases when you are starting from such a low number. In a small car +10mpg may only be a 30% improvement, while for you that same 10mpg would be 90% better. 4mpg is totally possible. Ford wasn't trying to make these things fuel efficient in the least, so there is all sort of room for improvement.