i used to have an old meter like that for my bike. it was a sensor setup by the wheel with a unit attacked to the wheel im guessing a magnet. to try something similar, try some sort of magnetic sensor possibly like that which people use for home security and run it through a frequency counter or a frequency to voltage convertor and run to a voltmeter. of course you'd have to do the calibrations yourself. i say scratch this idea and just go buy one they arent that expensive.