tachometer
/tæˈkɑmətər/
noun
- An instrument that measures the rotation speed of a shaft or disk, as in a motor or engine, usually in revolutions per minute (RPM).
- The mechanic used a handheld tachometer to check the speed of the spinning motor.
- He glanced at the tachometer before shifting gears to avoid over-revving the engine.
- The car's tachometer showed the engine was idling at 800 RPM.