A tachometer is a device that measures and displays the rotational speed of an engine, typically in revolutions per minute (RPM). It is commonly found in vehicles, such as cars and motorcycles, as well as on other types of machinery, such as jet skis, boats, and generators.

A tachometer typically consists of a dial or digital display and a sensor that is connected to the engine. The sensor measures the rotational speed of the engine and sends this information to the tachometer, which displays it in a readable format.

Tachometers are useful for a variety of purposes, including monitoring engine performance, diagnosing mechanical issues, and setting the correct idle speed. They can also be used to ensure that an engine is not being over-revved, which can cause damage to the engine and other components.

In the case of a jet ski, a tachometer can be used to monitor the performance of the engine and help ensure that it is operating within its optimal range. It can also be useful for adjusting the jet ski's speed and acceleration to suit the rider's preferences and the conditions of the water.