Speedo Facts

Reasons for Speedometer Error Tolerance

There are sound technical reasons for a car speedometer having a wide tolerance. The factors that influence the accuracy of the speedometer are:

	Diameter of the tyre
	Accuracy of the speedometer sensor
	Accuracy of the speedometer indication
	Ability to accurately determine the reading on the speedometer dial
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In modern cars, nearly all speedometers are controlled electronically. The car speedometer sensor is typically by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.

The device that measures the rotation is usually fairly accurate, typically 1 rpm in 1000, giving an error of +/- 0.1%.

Remember that there are two separate instruments involved in a car and an assumption:

• the measurement device in the gearbox measuring the rotating speed of the gearbox output shaft.

• the speedometer dial and indication. more...

• the assumption that the circumference of the tyre is constant. more...