Automotive tachometers measure the rate of rotation of the crankshaft of the car's engine, typically in thousands of rotations per minute (RPM). This can assist the driver in selecting the most appropriate throttle and gear settings that the driving conditions call for, or perhaps more frequently, allow automotive enthusiasts to admire how fast they are "revving" the engine.
Tachometers fitted to cars, aircraft, and other vehicles typically have markings indicating a safe range of speeds at which the engine may be operated. Prolonged use at high speeds may cause excessive wear and other damage to engines. On an analogue tachometer, this speed range is typically indicated by a portion of the gauge marked with a red line, giving rise to the expression of "redlining" an engine - ie, running it at (dangerously) high speed.
On many recorders, the tachometer spindle is connected by an axle to a rotating magnet that induces a changing magnetic field upon a hall effect transistor. Other systems connect the tach spindle to a stroboscope which alternates light and dark upon a photodiode.
The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is being played back at the proper speed. The signal from the tachometer is compared against a reference signal (either a quartz crystal or alternating current from the mains). The comparison of the two frequencies drives the speed of the tape transport. When the tach signal and the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll Sound!" A moment later the sound man calls back "Sound speed!" This practice is a vestige of the days when recording devices required several seconds to reach a regulated speed.)
Having perfectly regulated tape speed is important because the human ear is very sensitive to changes in pitch, particularly sudden ones, and without a self regulating system to control the speed of tape across the head, the pitch could drift several percent. A modern, tachometer-regulated cassette deck has a wow-and-flutter (as the measurement is called) of 0.07%.
Tachometers are acceptable for high-fidelity sound playback, but are not acceptable for recording in synchroization with a movie camera. For such purposes, special recorders that record pilottone must be used.
Tachometer signals can be used to synchronize several tape machines together, but only if in addition to the tach signal, a directional signal is transmitted, to let the slave machines know no only how fast the master is going, but in which direction.