James,
It may help to provide some other details about your drive train such as # of motors and diameter of your wheels. Generally, there is more to it than the sprocket reduction after the gearbox. To help you, JVN from 148 updates
his calculator every year. Play with the #'s so they match what's on your robot, and tell us what your final speed is. Be careful when gearing for high speed though -- there IS a trade-off.
Finally, don't let anyone tell you that 2 CIMs on a drive train is a bad idea -- if it's geared properly, it works just fine (and you can still be very successful).