A rev limiter is a device fitted in modern vehicles that have internal combustion engines. They are intended to protect an engine by restricting its maximum rotational speed, measured in revolutions per minute (rpm). Rev limiters are pre-set by the engine manufacturer. There are also aftermarket units where a separate controller is installed using a custom rpm setting. A limiter prevents a vehicle's engine from being pushed beyond the manufacturer's limit, known as the redline (literally the red line marked on the tachometer). At some point beyond the redline, engine damage may occur.

Limiters usually work by shutting off a component necessary for the combustion processes to occur, whether it be fuel, air or spark. Compression-ignition engines use mechanical governors or limiters to shut off electronic fuel injectors. A spark-ignition engine may also shut off fuel or stop the spark ignition and some just reduce the engine's power by changing the spark timing.

In the case of an automatic transmission in "drive" mode, the engine rpm stays safely within the range that the transmission chooses. Only when over revving the engine in "park", "neutral" or "manual" modes is there any need for a rev limiter. These vehicles often did not include a tachometer until the turn of the millennium. Without this gauge, the redline cannot be seen but there is so little risk of excessive engine speed with fully automatic transmissions that engine rpm is not a concern.

However, with a manual transmission engine rpm can redline in "neutral", or by shifting to a higher gear too late, or by shifting to a lower gear too early. In the case of "neutral" or shifting up too late, a rev limiter can easily keep engine rpm below the redline.

If a manual transmission is shifted down too early, the speed of the vehicle will drive the engine over the redline. In this case, a rev limiter will cut engine power but it cannot prevent the engine's rpm from going beyond the redline.

Perhaps the worst situation occurs when a shift is "missed". In the diagram shown, it is possible to be at high rpm and "miss" shifting from 2nd to 3rd and get 1st gear instead. This will result in exceeding the redline and there is nothing to prevent an engine from being severely damaged due to valvetrain failure or connecting rod failure. Using the clutch as quickly as possible may avoid engine damage.

Most small engines, such as on lawn mowers have a speed governor. As the rpm of the engine increases, the throttle plate in the carburetor is gradually closed, reducing the amount of fuel and air admitted to the engine, until the engine rpm is stable. If rpm drops below the desired value, the throttle plate will automatically open, admitting more fuel/air mix to the engine. Adjusting the throttle generally adjusts spring tension on the governor, which in turn allows the engine to run faster or slower, as desired. While the redline cannot be seen on most small engines, due to their lack of a tachometer, the risk of excessive engine speed is not generally a concern.

Fuel-cutting rev limiters are the most common in road cars because they wear less on exhaust components, particularly the catalytic converter. These systems usually lean out the engine's overspeed by shutting off the fuel injectors, and are the only practical system on diesel engines. This is less popular in high performance or racing engines due to high temperatures in lean operation and the lack of a catalytic converter.