R2 is there to make sure that the transistor turns off correctly and quickly when the input is low. It may not be needed, but it can't hurt.
Your relay coil may well produce spikes that trigger the rev counter, but there is a danger that you could damage the transistor, which is rated to 160 V or so. You should put a resistor in parallel with the coil. Chose a value about 10 times the resistance of the coil and you should limit the voltage spikes to about 120 V.
There is no guarantee that the tacho will read correctly once you get the correct signal through. The wasted spark ignition systems that I have seen used two coils for a 4 cylinder engine, so each coil was triggered at half the rate that a distributor system would have been triggered at.
If there is a single coil pack with 4 outputs, the 5 V signal to it could be just about anything.
You could also run the tacho off the AC output from the alternator. That usually needs opening up to add a wire to access the AC direct from the coils, before the rectifier, and the frequency won't be the same as the spark frequency, but it will work on just about any car.