Speedometer Tester Accuracy: Reading the Legal Tolerance Correctly
A speedometer tester compares the speed a vehicle's instrument indicates against the true speed measured on rollers. The tolerance it checks is deliberately one-sided: under UN Regulation No. 39 the indicated speed must never be less than the true speed, and may exceed it only within a defined margin. Understanding that asymmetry is the key to reading a speedometer result.
What the tester measures
The tester runs the driven axle on rollers and measures the true speed from the roller rotation, while the operator reads the speed the instrument indicates. The difference between the two is the indication error, recorded at the test speeds the regulation prescribes.
Measuring on rollers removes the road-test variables of traffic, gradient and surface, which is what makes the result repeatable and auditable for an inspection record.
Why the tolerance is one-sided
The speedometer accuracy requirement in UN Regulation No. 39 is asymmetric by design. The indicated speed must never be lower than the true speed, and the amount by which it may read high is capped by a formula that allows a fixed margin plus a fraction of the true speed.
This means a speedometer that reads exactly true is acceptable, one that reads low is not, and one that reads slightly high is acceptable within the cap. The tester's job is to confirm the vehicle sits inside that band, not to chase a zero error.
- Indicated speed must never be less than true speed.
- Reading high is allowed only up to a defined cap.
- The cap combines a fixed margin with a fraction of true speed.
Reading the result correctly
A pass means the indication is within the one-sided band at every test point. A fail means either the instrument reads low at some speed, or it reads high beyond the permitted cap. Both are worth distinguishing, because a low-reading instrument is a legal risk and a high-reading one is usually a calibration or tyre-size issue.
Tyre size matters here: a speedometer calibrated for one rolling radius will read differently after a tyre change, which is exactly the kind of drift a periodic check catches.
What to confirm about the tester itself
The tester's own accuracy must be better than the error it is trying to find, and the reference speed measurement is itself subject to a tolerance. Confirm the station's calibration is current and that the rollers are driven at a controlled, known speed before trusting the result.
Keep the calibration and the test speed on the report, so a result can be tied to a method and a traceable reference rather than a bare pass or fail.
Common questions
Why can a speedometer read high but not low?
Under UN Regulation No. 39 the indicated speed must never be less than the true speed, because a low-reading instrument would let a driver exceed a limit without knowing it. Reading high is allowed only within a defined cap that combines a fixed margin with a fraction of true speed.
What test speeds does a speedometer test use?
The regulation tests at defined points such as 40, 80 and 120 km/h depending on the vehicle's maximum design speed. The tester runs the axle on rollers and compares the indicated speed against the true roller speed at those points.
Does changing tyre size affect speedometer accuracy?
Yes. The speedometer is calibrated for a rolling radius, so a tyre of a different size changes the relationship between wheel speed and road speed. A periodic check catches exactly this kind of drift.
Need the equipment side of this?
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