In vehicles, the speedometer tends to “lie” upwards intentionally, meaning it shows a speed slightly higher than the actual speed.
In practice, for most vehicles, the error is between 2% and 7% upwards, depending on the model, tires, and speed.
The main reasons for this:
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Legal and Regulatory Requirements – According to international standards (such as in Europe – the UNECE R39 standard), vehicle manufacturers must ensure that the displayed speed is never lower than the actual speed. The gauge can show up to about 10% + 4 km/h more, but never less. This is intended to prevent a situation where a driver believes they are traveling at, for example, 90 km/h, but is actually traveling at 100 km/h and breaking the law without knowing.
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Tire Variance – Tire diameter affects the calculation. Over time, the tire wears down and the diameter decreases slightly, which increases the calculated speed. Additionally, people replace tires with different (but approved) models, and this changes the reading. The upward deviation takes this into account.
From the European Standard:
Section 5.4:
“The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.3.5 above, there shall be the following relationship between the speed displayed (V₁) and the true speed (V₂):
0 ≤ (V₁ − V₂) ≤ 0.1 V₂ + 4 km/h”