Dave;
It's not as simple as a direct RPM to Voltage correlation...that would make it too easy!...it's an RPM to Current correlation, because it drives a current meter (which I expect has a nonlinear response on top of it...).
...so it sounds like you have a fairly unique gauge...again, if your Tach was the type where gauge was driven by remote module, you have the option of replacing the module with a modernized one...
Mike;
The "generous Tach" is a common age of electrolytic capacitor/calibration potentiometer related symptom, indeed within the gauge electronics.
Cheers
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