Which device is used primarily for measuring engine speed?

The tachometer is the device specifically designed for measuring engine speed, typically expressed in revolutions per minute (RPM). This instrument provides crucial information about how fast an engine is running, which is vital for tuning, diagnostics, and performance evaluation. Accurate RPM readings allow mechanics to assess engine performance and make necessary adjustments to ensure the engine operates efficiently.

In contrast, a multimeter is primarily used for measuring electrical parameters such as voltage, current, and resistance, which is not directly related to engine speed. A differential pressure gauge measures pressure differences in fluids or gases, useful in various applications but not in measuring RPM. A caliper is a tool used to measure dimensions with precision, typically of physical objects, and does not serve any purpose in assessing engine speed. Thus, the tachometer is the clear choice for accurately gauging how fast an engine is operating.