A tachometer is used to measure the speed of the crankshaft in the engine of a car and is stated in Revolutions Per Minute (RPM). Knowing the engine speed can help the driver select the most appopriate gear for the road conditions. There are two main types these are inductively coupled and direct coupled.

Inductively Coupled

These tachometers use a the magnetic field generated by the car ignition system to generate the pulses to drive the tachometer.

Direct Coupled

Direct coupled tachometers are connected to the ignition system itself. The pulses generated for turning the ignition coil on and off are used to drive the tacho.

The Gauge

The gauge itself simply measures the speed that the pulses are generated by the engine. There are two factors that will govern this which is the number of cylinders and the engine speed.

The gauge itself operates by averaging out the pulses. When the pulse rate (frequency) is low the average is low and the needle will be closer to zero RPM. As the engine speed rises the average will rise moving the needle away from zero RPM.

Fundamentals of Motor Vehicle Technology
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