Probably get 17 or 18 with the cruise set on 65 on level ground.

Switch to a 235 75 15 tire will help a lot and be cheaper to buy, those 31-10.50 make a lot of drag.

Get a set of the orange 4.6 ford fuel injectors at the JY for a little MPG bump.

a 205 t-stat helps.

I am not causing global warming, I am just trying to hold off a impending Ice Age!