And for an alternative method that uses your PC instead of a bike computer, check out Instructables. The Wodometer(TM) lets you easily measure how far and how fast your critter is going. The Wodometer consists of a Sigma BC500 Bike Computer pre-calibrated for a Wodent Wheel and a sticky-back magnet for mounting on the back of a wheel.
For a little more detail, you can see the instruction sheet that's included with the Wodometer: Instructions.