A tachometer measures engine ___.

• A. Displacement
• B. RPM
• C. Torque
• D. Horsepower

Answer: (B)

A tachometer measures how fast the engine is turning, specifically the rotational speed in revolutions per minute (RPM). It’s a speed indicator for the crankshaft, helping you keep the engine in a safe and optimal operating range and to set idle or shift points in some systems.

Displacement refers to the engine’s size or volume, which a tachometer does not measure. Torque is the turning force produced by the engine, which depends on many factors and RPM, but the tachometer doesn’t directly measure torque. Horsepower is a calculated power figure that comes from torque and RPM (horsepower = torque × RPM / a constant depending on units). Since the tachometer only shows RPM, it’s the best choice for describing engine speed rather than size, turning force, or power.