Two things to consider, the number of pulses and the voltage (or current?) of the signal. The Diesel tach counts the number of flywheel revs through an inductive pulse counter. IIRC there's only one lug on the ring gear. The gasser tach counts the number of cylinder firings from the coil and divides by the half the number of cylinders. Unless there is some intermediate device to take these differences into account, you won't get a proper display.

Sixto
87 300D