JVN of team 148 created this great drive train/ mechanical calculator.
http://www.chiefdelphi.com/media/papers/2059
We have gone with a 20:1 total gear reduction (including wheels) in the past with 4 CIM motors. Its more of a medium speed at about 8-9 feet per second. Its kinda nice for newer drivers because its not too fast and has decent pushing. We liked it because it has enough torque spin the andy mark FIRST wheels when it crashes into something. Kinda handy with a run away robot driving into a wall in autonomous because you won't trip the circuit breaker right before you start a the tele-op part of the match.
This year we are going for a much quicker robot. We will see if our driver likes it. We are keeping extra sprockets handy to tweak with the gear ratio.