[Al Skierkiewicz]

I can only answer your question with this observation. I have seen many teams run too low a gear ratio in an attempt to have a high speed robot. In operations where the robot must do some real work, the motors are running near to stall which has the highest current demand. This will always mean tripping breakers and burned motors and speed controllers. Determine a realistic top speed (we could cover 48 ft in about 10-12 seconds although I have heard some teams clocking as high as 12 ft./sec) calculate your drive wheel RPM for that speed using the circumference of the wheel you intend to use. Decide which motor to use for drive system and then calculate the gear ratio needed to achieve that. Many teams use either the drill motors or Fisher Price motors for drive.
If you plan on using the drill motors, remember there is a locking pawl in the transmission you may want to remove. This locks the output shaft when you remove drive power to the motor.(part of the screwdriver mechanism) This results in almost certain failure of transmission housing.
If you plan on using the Fisher Price trasmission, remember it was intended for kiddie cars to not go fast enough to damage a small child. It also needs some modification that has been discussed elsewhere on the forums to beef up one of the shafts and improve efficiency.
I hope this gets you started, since I am an electrical guy and can't help much more than that.
Al