Speedometer deviation: why it exists and how the indicated speed can vary

The speedometer does not indicate actual speed. The speed indicated by the needle or on-board instruments cannot never be less than the real one. The speedometer of a car, motorbike, scooter or bus must not indicate a lower speed than the actual speed. For example, it cannot happen that the speedometer shows 100 km/h when the car is traveling at 105. This is required by specific legislationderiving from Unece regulation 39 (the UN European economic commission which also supervises technical regulations) which all manufacturers must comply with.

SPEEDOMETER REJECT

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The regulation R39 defines the requirements for the approval of category L, M and N vehicles, i.e. motorcycles/scooters, cars and heavy vehicles with reference to speedometers and odometers. While there are no indications on the precision of the latter, on that of the speedometer the rule is very precise: the vehicle must indicate a speed tainted by a certain margin of error, never lower than the real one (at most it can be identical) and no higher than a certain threshold. The waste admitted is between the real speed and the real speed plus 10% + 4 km/h.

CONCRETE EXAMPLES

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Here are some practical examples, valid for new or recent cars of every category, manufacturer and price:

• at a real 50 km/h the speed indicated by the speedometer must be between 50 and 59 km/h;
• at 70 km/h real, between 70 and 81 km/h;
• at 90 km/h real, between 90 and 103 km/h;
• at 110 km/h real, between 110 and 125 km/h;
• at a real 130 km/h, between 130 and 147 km/h.

THE RATIO

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But what are the reasons so the regulator has imposed that the speed of vehicles must be overestimated (within a certain limit) but never underestimated? First of all for safety: if the speedometer indicates a speed lower than the actual speed, the driver could exceed the limits unknowingly, risking accidents or fines. And then for technical issues: the speed shown by the instrumentation is influenced by factors that can vary over time, physical factors (tyre pressure and wear or rim diameter) and/or environmental factors (temperature or vehicle load). The speedometer deviationtherefore the margin provided for by the regulation serves to guarantee that, even when the parameters that influence it vary, the speed shown to the driver is not lower than the actual one.