>On some cars I've seen magnets on the drive shaft
>with a  pickup for the cruise control, but the
>tachometer seemed to be driven from elsewhere.

Well for cruise control, you obviously need to have the pickup after the
gearbox. Same as for a speedometer.

Electronic tacho is generally generated from whatever signal is used to fire
ignition pulses in a petrol car, do not know how it is done in a diesel,
unless there is electronic control of the injectors. If the tacho signal is
used for cruise control, then there needs to be some interlink with the
gearbox to know what gear it is in, but there is still a problem if there is
a torque converter involved.

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