Buying a trailer just because he wanted to tow it across the US? Glad you talked him out of it. You buy a trailer because you need it (or at least that's what I thought!)
There are different gearsets you could put in; generally, gears that give you more power(climb/4x4) allow the engine to run faster to generate more power, and hence use more gas. Highway gears let the engine run slower, in theory saving gas (but with big tires and highway gears you actually use more, so there's a break point somewhere depending on the vehicle)
In any case, it would probably be more money to change the gears than it would be to just drive the truck as is.
Now you can ship the truck and drive your car. That'll really save gas.
