Last summer I built a speedo calibrator which puts out very accurate rates of pulses per second to simulate the pulses normally supplied by the rear wheel pickup. I have been using it and have drawn up a finished schematic which I will send to anyone who wants it.
Originally, I measured the distance the rear wheel travels, etc. to determine how many pulses per second causes the speedometer to register a mile an hour. I think I was within about 1 MPH at 75 MPH.
Not bad, but David Weiszbrod of Fuel Plus fame, supplied the fact that 75 pulses are input to the odometer to make it register 1 mile.
Bill Heckel supplied me with a data sheet for the chip inside the speedo and I determined from the schematic I drew of the circuit inside the speedo, that the chip divided the pulses from the rear wheel by 64. Working backwards then 64 X 75 pulses/mile = 4800 pulses/mile from the rear wheel. Then 4800 pulses/mile X 1 hour/3600 seconds = 1.333 pulses/sec for each MPH the speedo displays.
My box puts out 33.3, 50, 100, and 166.6 pulses per second as well as some other not useful rates (accurate to +/- 5 parts per million!). This corresponds to 25, 37.5, 75, and 125 MPH. I wired mine to put out 37.5 and 75 MPH selected by a momentary toggle switch which also applies the power from an internal 9 volt battery.
You can get the Epson 8651B osc/divider chip to build it from Digi-Key, 1-800-344-4539, for about $20 plus shipping. The rest of the parts can be gotten from Radio Shack.