If you really want another gear and have the money, look into a gear vendor unit.I have a 95 psd f350 dually 5 speed 2x4 w/ 4.10 rearend.
4" Exhaust no muffler
My wife got 19mpg last week (before tymar) driving to work and back cruising 50-55mph.
ONce i speed past 55-60, mpg goes down (it seems like exponentially) with increasing speed.
It would be nice to have a sixth, or higher gear.