I got this from Samstain's R/C Speedo page
This thing actually works! I just have to fit it into my car now. I just tested it by putting the slotted disk on a dremel and spinning it. It will stop measuring at 85mph which should be plenty for my mirage. I think the limitation is the IR gate. I haven't experimented with different slot sizes on the disk though. I will post the results on how I attached the disk and the speeds I measured as soon as I can.
Something to keep in mind if you're building this is to make sure the speedo will work. Check by multiplying your car's wheel circumference by 10 and see if you can enter that. Mine only accepted 3000mm max and I needed 3400mm for my V-spec. So I decided to enter 1700mm and the displayed speed will be exactly half of the real speed. You can also mess around with the divide ratios. Instead of 20 maybe you can do 10. I didn't do this because I had already glued and soldered mine together.
Note that you can substitute the transistor for a more common one like the 2N2222 which is available at Radio Shack. I think the transistor and the chips all went for around $1 each. The IR gate was $2-3 I think. You can pick up a $20 bike computer at any bike shop. Or if you have a bike you can just borrow its computer and buy an extra mount (the mount for my Performance Axiom 8.0 is $10). Interestingly, the performance computers are actually made by the German company VDO. Yes, it is the same company that makes the gauges for many full size cars. Good luck!!!!