What instrument is used to measure engine speed?

The tachometer is the instrument specifically designed to measure engine speed, commonly expressed in revolutions per minute (RPM). This measurement is crucial for drivers, particularly in heavy vehicles, as it helps them understand how hard the engine is working and when to shift gears for optimal performance. By monitoring the tachometer, drivers can avoid over-revving the engine, which may lead to damage or excessive wear, ultimately contributing to better fuel efficiency and engine longevity.

In contrast, the speedometer measures the vehicle's overall speed, the fuel gauge indicates the amount of fuel remaining in the tank, and the temperature gauge tracks the engine's operating temperature. Each of these gauges serves a different purpose and does not provide information about engine speed, which is why the tachometer is the correct choice in this context.