[G6Tanuki]

A normal DMM will be assuming something like a sinewave at its input, the waveforms from a loosely coupled input from a points/condenser classic Kettering ignition circuit come with loads of complications, which it is not competent to interpret.

In days past I built a bunch of digital revcounters from circuits in Practical Electronics. Elektor etc.. They never really worked well... 'points bounce' invariably caused misreading at high RPM, and we learned to divide the displayed RPM by two to compensate.