RioRand 0.56" Digital Motor LED Tachometer RPM Speed Measure Gauge Meter Tester 60-9999 (Blue)

Even though mine sorta worked, it is definitely going back. But for what it's worth... Looks like it reads revolutions per second, with a range between 5Hz to 135Hz. Within that range, it consistently reads 18% higher than the HP frequency counter. It is not as advertised.

You get what u pay for really crapy no instructions cables in the back r so thin they make bad connection go buy a burger instead waste of money.....

Only goes to 999 RPM. Last digit doesn't light up. I'm sure if advertised correctly no one would buy it. From an electrical engineer.

Bought 1 that died on me in 5 minutes, thought I shorted something. Bought another and installed exceedingly carefully and it died after 10. Money down the drain.

Only 3 of the 4 digits light up. I wrote the seller. We'll see if I get sent a replacement.