Which device is used to measure speed accurately?

The tachometer is designed specifically to measure the rotational speed of a shaft or a disk, typically in revolutions per minute (RPM). It provides direct and accurate readings of how fast an engine or motor is operating. This precision makes it an essential tool in various applications, especially in automotive and mechanical engineering contexts, where knowing the exact speed of a rotating component is crucial for performance tuning and safety.

In contrast, while a speedometer measures the speed of a vehicle, it may not provide the same level of detail or accuracy in different contexts as a tachometer does for rotational speed. A dial indicator and a dial gauge are instruments used mainly for measuring linear displacement, thickness, or surface deviations rather than speed, making them unsuitable for this specific purpose. Therefore, the tachometer stands out as the appropriate device for accurately measuring speed.