Don't listen to the naysayers, true it will not be a hot rod but you never said you wanted one. I will also tell you I hate fords but the 4.6 aint that bad, not great but not bad and you gotta consider he said he mabey averages 40mph so aero dynamics is not going to play a big part.

I think 20mpg is easily possible if you are not running pedal to the metal all the time.

2 benifits you will have over the bigger motor is pumping losses will be significantly lower and friction will be a lot lower.

As for the compression comment I should have presented it with more other input. Yes you want as much compression as it will handle without detonateing itself to death but in a heavy truck with super short duration cam and small cubes you will find that limit at a lower compression than a bigger motor with a bigger cam. I would shoot for something like 9.5 max and be prepared you might need premium fuel.