about a mechanical device he was going to fit to his car for rallying purposes

Well, I did install that (a 3-wire hall-effect sensor epoxied inside a threaded brass tube) and it works great, but it has no intelligence of its own -- you'd have to program something to interpret pulses. We epoxied 2 magnets to the axle shaft, so that if 1 broke off, we could still reprogram the rally computer to 1/2 the number of pulses.

So, I now understand why Frank wants the RPM sensor, but am trying to figure why Tony wants a fixed speed sensor. Mute audio output when speed > 140?