If you do a lot of highway driving then you might consider an overdrive unit, they used to add on behind the transmission. Back when I hauled campers over the road the unit was built by Doug Nash Overdrives. This seems to be the main one now. It is switchable, and looks to change the gearing 25-28%. You can get it as an over or under.
Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions.