I know that speed = distance/time. I was thinking, since this is going on my bike, I could measure the circumference of the rim and hook up a circuit that would have a something (motion detector?) on the front brakes and another something on the rim, the one on the brakes counting the number of RPMs. I could have it hooked up to a timer, and it would have a digital read-out, which would calculate the speed, that updates every certain number of RPMs.
Has anyone ever built a speedometer, or if not, do you think something like that would work?