Mike, awesome tool. Without an electronic engine, that's probably the most accurate way to measure RPM, and it's cheap!
$24 and it's eligible for Prime.
https://www.amazon.com/Neiko-20713A-Digi...RydWU&th=1
About this item
TACHOMETER VS. SPEEDOMETER: While a digital speedometer will read the current speed of a vehicle, this automotive tachometer is used for measuring the rpm of a revolving shaft, as with the driveshaft of an automobile.
ACCURATE RPM METER: This handheld tachometer is highly accurate and noncontact. With high-speed laser-aim operation and instant readings from 2.5 to 99,999 rpm, this radar gun gives you accurate rotational speed readings from a distance.
AUTOMOTIVE TACHOMETER GAUGE: This big- or small-engine tachometer measures velocity in revolutions per minute. The information is displayed on a large and easy-to-read LCD screen.
TECHNICAL SPECIFICATIONS: This pocket radar device is accurate to +/- 0.05% and is capable of measuring at a distance from 2" to 20". It measures revolutions over a 1-second sampling time and stores them as the last, minimum, or maximum reading.
COMPACT DIGITAL COUNTER: The compact, lightweight design of this photo tachometer makes it easy to get speed readings for diagnostic and measurement purposes. This device operates on 3 AA batteries (included).