A little google-fu returns this:
Quote:
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Originally Posted by t-inter'web
Modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display, the latter of which is more common nowadays. Pulse counts may also be used to increment the odometer.
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But that doesn't answer the specifics, I know, merely confirms what was already said.