There are sound technical reasons for a car speedometer having a wide tolerance, particularly under ADR 18/02. Unlike the new ADR 18/03 which applies to vehicles registered after 30 June 2006, ADR 18/02 does not provide any test qualifications to its required error tolerance of ±10%, consequently there is no known base calibration point.
It is important to note that there is a fundamental difference between speed measurement by a device such as police radar and a car speedometer.
The police radar device is measuring the time taken by an object to move between two points, and within the accuracy of the device, calculates a speed.
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1. Diameter of the tyre due to tyre wear.
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New tread depth is 10mm, legal minimum is 1.6mm. Difference of 8.4mm changes the tyre circumference by an average of 2.9% |
3% |
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2. Diameter of the tyre due equivalent tyre sizes |
Alternative tyre sizes of different widths and profiles are permitted to be fitted to the car. These can alter the circumference of the tyre by up to 3% |
3% |
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3. Diameter of the tyre due to tyre pressure |
On a 215/45R17 tyre, altering the tyre pressure from 22psi to 40psi alters the height of the tyre and consequently its circumference |
2% |
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4. Diameter of the tyre due to load in the car |
On a 215/45R17 tyre, with a tyre pressure of 22psi, adding 60kg for a full tank of fuel, 150kg for two adults in the rear seat and 60kg of luggage alters height of the tyre and consequently its circumference |
2% |
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5. Accuracy of the speed sensor on the gearbox |
Typically a device that measures measures the rotation in the gearbox. It is usually fairly accurate, typically 1 rpm in 1000
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± 0.1% |
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6. Accuracy of the speedo dial indicator |
If a specialist Speed Detection Speedo is only accurate to +/- 2 km/h accuracy and displays in 1km/h increments, what makes you think your speedo is equal or better?
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± 3% |
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7. Resolution of the speedo indication |
The graduations on the speedometer dictate the degree to which the speed can be resolved.
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± 2% |
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8. Eye sight, ability for the eyes to focus on the speedometer. |
It is a medical fact that in people 40 years and over the eye starts to lose it's flexibility due to age and consequently the ability of the eye to rapidly refocus, or to refocus on close objects diminishes. The error will depend on the resolution of the indication and is likely to be twice the width of the speedo needle. |
± 2% |
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TOTAL Error |
There are two errors to summate:
Total of the absolute errors (1,3 & 4) is 8%. These give 8% variation in the car speed, independent of instrumentation errors.
Total of the tolerance errors (5 & 6) is ±3.1%. This is the accumulated error of the instrumentation.
The readers ability to determine the reading (7 & 8) adds another ±4%.
Total possible error to determine the speed of the car will be:
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20% or ± 10% |
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Note: the error due to alternative tyre sizes (2) is not included in the total above. This would increase the total to 23% or ±11.5%. |
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