But only for some Tachs...
90% of the drivers we sell work the tachs perfectly fine with the suggested modification. The biggest problem are the early tachs such as these 3door ones as we havent had a set to work out how to modify them to accept a lower signal.

I am informed that the diode circuit above will work fine on the 3door tach, but i have yet to try it myself, although i did try it on an escos and it wouldnt work. Worth a try for the cost of the components.

Alternative remedies.
Use an Autometer tach signal doubler.
Fit an aftermarket counter.
Send clocks to a place who modify Tachs to wasted spark config.