[Alan Anderson]

The usual way to do this is to put rotary shaft encoders on the drivetrain. Sometimes they're connected directly to the gearbox, sometimes they're driven by an extra sprocket on the chain powering the wheels, and sometimes they're on a "follower" wheel that rides on the carpet. The electrical output from a typical shaft encoder is a pair of digital signals that change in sequence to indicate both speed and direction of rotation. Search the ChiefDelphi forums for "encoder" if you want to find more information, and ask again when you have specific questions.

(The AndyMark Super Shifter has a built-in spot for such a device on the output shaft.)