Engine Speed (Rpm)

It is shown on the tachometer. Idle speed is usually below 1000 rpm. Each engine has a maximum speed. Diesel engines usually have lower maximum speeds than petrol engines because of their heavier internal parts and slower combustion. Modern automatic gearboxes choose gears to keep the engine in an efficient speed range, and the rev limiter cuts fuel or ignition if the maximum is reached.

The crankshaft sensor measures the speed.

Unusual fluctuations can point to faults. Very low speed under load strains the engine.