Tach not interchangeable

I recently exchanged my 1994 diesel instrument panel (km speedometer) with an American panel but it was a gasoline instrument panel. I would have thought that the ECU on these trucks processes RPM info by the same pulses to the tach by the time it gets past the ECU but mine reads hideously high. What do I have to do to get the American panel to read the correct RPM? Everything else reads perfectly.

It will need a buffer to adjust the frequency. The gassers don't use the alternator for the signal. You can make your own, or buy one. More info on the DIY HERE.