UNI-T UT373 Mini Digital Tachometer, 10~99999 RPM Non-Contact Tachometer Speed Meter Speed Gauge with Backlight, Max/Min/Data Hold, Rotating Speed/Count Function Switching: Amazon.com: Industrial & Scientific
LEGO Speed Computer RPM Test - YouTube
Nettigo: Geekservo LEGO® Compatible DC Motor [Kittenbot]
Exploring Piston Engine Design With LEGO | Hackaday