A tachometer measures rotational speed in rotations per minute, and in an automobile, it measures the speed at which the crankshaft in the engine rotates. Knowing the speed of the rotation helps drivers using a manual transmission to adjust their rate of acceleration to avoid engine damage.
When drivers operate a car with a manual transmission, they are responsible for shifting gears at the appropriate time. It is possible to listen to the sound of the engine and gauge the proper time for shifting, but if the tachometer indicates the RPM is at or over 3,000, it is time to shift into the next gear. Drivers in the highest gear should keep their RPM closer to 2,000 for optimal engine performance.
Sometimes RPM climbs well above 3,000, as when sudden acceleration is necessary. This does not damage the engine during the short burst of time it takes to enter a highway or pass another motorist, but the driver should not operate a car at that level under normal circumstances. Cars designed for racing operate at higher RPM for longer amounts of time, but the engines, components and even fluids such as the motor oil are different to compensate for the additional strain.