I agree with circuitous. Most tachometers just take a pulse train from the ignition, so all you need to do is vary the pulse frequency instead of the pulse widt

I agree with circuitous. Most tachometers just take a pulse train from the ignition, so all you need to do is vary the pulse frequency instead of the pulse width. On my Honda NC30 project bike, I'm feeding the stock tachometer with a variable-frequency pulse train from an Atmega328. The tacho just wants its input pulled to ground with a frequency equal to 4x the RPM, so one external transistor is all that's needed. (The 328's main function is for talking to the wideband O2 sensors and sending that data over the CAN bus to a Microsquirt ECU, but it works nicely for also reading engine RPM over CAN and outputting the signal to the tacho.)