Hi Helmut,
There are easier ways than picking up from a high tension spark plug wire, your copper will work like a capacitor plate and pick up a signal, or should I say signals, very ugly waveform, then you will need to condition it etc etc . . . Better I think to use some other method, like optical pickup or magnetic pickup. I did not recommend picking up from the primary circuit for similar reasons as the secondary, namely too much voltage, which you have to turn into a TTL level signal. I would have the tach count more than 1 pulse per revolution so as to promote stability at low RPM, 4 seems like a good number. Then RPM = (counts per second / 4) * 60