Calibrating tachometers?

KitCarlson said
Full wave rectify it to get 120Hz for 1800 RPM.
.

I thought of that, but 1800 still doesn't give one a very comprehensive idea of how linear the meter is

KitCarlson said
. Unless you run an amplifier and transformer. Beware most amps will not like the high inductance of the transformer and might be damaged.

I'm thinking a 1K or so resistor across the transformer output to the tach should tone it down, no pun intended.

I was only getting about 20V AC (RMS) so peak to peak is a mite higher