Is this in a modern or classic? There is not a variable voltage with RPM anywhere in an older car. Variables are mostly resistance values, ie fuel senders. whats it for? Sounds like something that could be made to read the rate of a flying magnet or hall effect off a more modern damper and convert that to a voltage. You could put a meter on an electric tachs innards to find a variable voltage but I dont know if that is a voltge that makes the needle climb or a signal/time counter.