I just put a basic magnetic pickup tachometer on the tailshaft of this 13" DC forklift motor.
I wanted to know how many RPM it would spin at 12V.
Turns out that it's around 1200 RPM, or about 100 RPM/Volt.

An old tractor like this is designed to run at 1800 RPM or so for the PTO to spin at the correct speed. If I wanted to use this motor and get the PTO speed right, in theory, I could run it at 36V and 50% PWM.
When NOT using the PTO, it could be run faster.

This is the tachometer I'm testing out: https://amzn.to/3uR16qs