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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 tire Diameter of the tyre.
Accuracy of the speedometer sensor Accuracy of the speedometer sensor.
Accuracy of the speedometer indication

Accuracy of the speedometer indication.
accurately determine the reading Ability to accurately determine the reading on the speedometer dial.
accurately determine the reading Return to Summary

Accuracy of the Indicator

Remember that the speedometer in a car is a commercially produced product with limited accuracy. Cost is measured in $10's compared with $10,000's for a speed camera. (TAC web site advises that in 1990, 54 speed cameras were introduced at a cost of $4.5 million, i.e. $83,000 each, extrapolate that to today and compare it to the cost of a car, let alone the speedometer components.)

The accuracy of a radar measuring device is stated to be no greater than 2km/h, or ±2% at 100 km/h. A speed camera is stated to be 3 km/h or ±3% at 100km/h.

The accuracy of specialist speedometers made for police cars are rated at ±2% (these are still subject to errors due to tyre circumference change).

Most speedometers are a needle rotating through an arc of 180-270 degrees, it is a mechanical device and subject to error. Assuming an error of ±2o in the rotation of the needle across the dial this would be an error of 4o in 270o or 2.96%.

The error of the speedometer dial is combined with the sensor in the gearbox to produce an error for the instrument from detection of rotation speed of the gearbox output shaft to the indication on the dial.

This calculation returns a calculated error of ±3.1%, which is probably a little ambitious considering specialist speedometers made for police cars are rated at ±2%.

Speedo Graduations

The Road Safety (Vehicles) Regulations 1999 and the Australian Standard Design Rule 18 – Instrumentation referenced doesn’t stipulate speed graduations.

In many vehicles the speedometer is graduated in 10km/h increments and the speedometer needle width can represent 2-3 km/h.

The photo here is from a Surabu Liberty 2005 model, which does have an indicator needle which is 2.2 km/h thick. This significantly hampers the drivers ability to determine whether they are 0 or 3 km/h above the limit.

The error stated against this indication is ±2%. This is then combined with the error in the rotation of the needle to produce the overall error of the indication.

Here is a link to a Data Sheet on a Speed Detection Speedo, a digital speedometer designed for use in Police and law enforcement vehicles. It is only accurate to ±2 km/h and displays in 1 km/h increments.

Reading a rotating needle with 10km/h graduations can not be more accurate than a digital read out.