[Joel J]

AndyMark sells single speed transmissions for a reasonable price ($98), and the upgrade pack to use two CIMs on the kit transmission is $42.50 from banebots.com . I don't know how well either of these options work, but you can do a forum search on them to see what others think.

If you are just using two motors, then you probably can't gear the robot for too high a speed, or use too grippy a tire, unless you don't care about breaking your wheels loose before you stall the motors.