This is going to require some more effort on your part to properly spec the motors. Motors with gear boxes provide a certain amount of torque that is then transferred to the wheels. The wheels then transfer that torque into speed, and how fast you go is a function of how much torque you have and the diameter of the wheel.

IIRC, the relationship between the force that the wheels provide the robot and the torque that the motor provides the wheels is F = t/r, where r is the radius of the wheel and t is the torque of the motor. Using the stall torque that you see in the picture you provided you get the *maximum* possible force that can EVER be applied by your robot for a given wheel radius.

Also there is, unfortunately, an inverse relationship between wheel radius and speed - smaller wheels will make the robot go slower, but will drive it with more force for a given torque (huh? Well remember that the speed of the robot will be related to the rpm of the wheel and the radius of the wheel. At the same RPM, a bigger wheel will be moving faster at the rim!).

Good luck!

MIKE

ps: Technically you should have posted this in the "mechanics and construction" sub forum, since this is the electronics forum and you might have better luck there!

[EDIT]: wrong about which forum this was posted in.