A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common.
A tachometer that can indicate up to 7000 RPM (left)
Cessna 172's G1000 tachometer (1,060 RPM) and engine hours (1736.7 hours)
A tachometer on a tractor, reading up to 3000 RPM, with the hour meter below it showing 772.9 hours. The mark at 2500 RPM is the engine speed required to run the power take-off at 540 RPM.
Revolutions per minute is a unit of rotational speed for rotating machines.
One revolution per minute is equivalent to 1/60 hertz.
A car's tachometer marked in multiples of a thousand rpm