[Rickpt]

I have a photo tach, works on light bouncing or reflected back from either a white mark or reflective tape on the target. I used it to calibrate my speed control pot I have mounted remotely
on my CNC machine. I think I got it from Harbor Freight a few years ago. Its pretty accurate as I have checked it again a known RPM. Its possible your motor speed is varying and the read out is not
showing it because its mechanical.

https://www.harborfreight.com/digita...ter-66632.html

I checked the RPM with a tach I borrowed and it was rock solid,no variation when running.