I am considering installing an GearMaster UNDER/OVERDRIVE on my F350 PowerStroke with a 6 Speed Stick, and was wondering if anyone has any experince with this unit. What type of fuel Mileage improvements can be expected. Any comments or recomendations would be appreciated. Considering the rather expensive price of this unit, it would seem that the payback on this unit would be rather lengthy. I don't tow my trailer (an 39' King of the Road) that much, but when I do it is usually in the NC Mountains. I have found the gap between 2nd and 3rd to be too much and thought that the underdrive feature would be very beneficial as well.