i just got a new speedometer one with a rev counter, had the same problem, but one of my tutors (mechanic) told me that you might be able to get a pulse from the ignition or something like that.. ive mad a few calls and am on a mission to get it sorted.. will post something on here when its done. meanwhile.. good luck. J
You should note that if your car have a distributor, then the pulse from the primary
(NOT SECONDARY!!!) would be the one you need.
But if your car have the waste spark semi-electric ignition system, then the pulse form the primary
should be doubled then feeded to the counter, it can be achieved easily with a PLL, however im sure that you are not wish to do this.
So you may need to adjust the circuit of rev counter to adapt this.