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Assuming that you don't need much current for a signal, put the 6 volts through a 10K resistor, and then connect a 2.8-3.0 volt Zener diode from that to ground. A 1N4148 diode in series with the 10K resistor would not be a bad idea in case of a failure (don't want anything feeding back to the 6 volts).
It might not be a good idea to use a 317 regulator for this because its response time might not be that good and it's "instantaneous" regulation isn't very good without capacitors-which will smear and distort the signal that you want.
This assumes that the needed signal is "digital" - that is, a number of pulses per second, and not analog. My understanding of an electronic tach is that it is effectively digital.
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