[suicideneil]

Simple, the motor is only capable of pushing upto a certain 'load', that is, a certain weight of vehicle with any given gear ratio. The Revo would be the heaviest of those setups you suggested, hence you can only gear so high before the motor starts drawing too large of a current = fried esc and cooked motor. The lighter the vehicle, the higher you can gear because there is less of a phyiscal burden on the motor. Same reason why having lightweight wheels & tires is better, since the vehcile will excellerate quicker and put less of a load on the motor = less rotating mass (fly wheel effect).

If you want the truck to go faster, you need a larger motor like a Neu 1527 or something comparable, basically something capable of producing much more torque which allows for higher gearing for the same current draw, or something along those lines anyway.