Gilbert,

This is not an easy task: you cannot really modify the wheel - like drilling holes or cutting teeth - without affecting the measurement itself. You cannot use any mechanically coupled method, nor magnetic-based method either.

The only thing I would try to do is to paint half of the wheel mat black, and build some sort of optical reflection sensor. These can be bought ready made, but the good stuff is really expensive. You can experiment though, maybe for start pick two LEDs, modulate one to say a kilohertz or so (e.g. from a 555 or even a '51 if you know already how to do it), and use the other as a photodetector - build an amplifier/filter/rectifier/schmitt combo from a cheap quad opamp (LM324 are popular in this kind of application). The mechanical setup is really important - you want to pick up only the light reflected from the wheel and not directly from the transmitter.

I hope it is enough to have a resolution equivalent to one (or half) revolution of the wheel. You maybe can get down to a quarter of that, but not really much better.

Once you get a reliable readout method, the rest is a piece of cake, given the '51 expertise in this Forum... :-)

Good luck!

JW