Learn why blindly trusting the speedometer can make you go slower than you think — and why it's a good thing
It’s a common scene for those who use navigation apps: while the dashboard speedometer reads 100 km/h, Waze or Google Maps indicate 95 km/h or 96 km/h. This discrepancy is not a system error, but rather a technical and regulatory characteristic of the automotive industry.
Generally speaking, GPS is the most accurate instrument for measuring actual speed at cruising speed. This is because the system calculates speed by measuring the distance traveled over time through satellite tracking, regardless of the mechanical parts of the vehicle.
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Speedometers, on the other hand, work by measuring the rotation of the wheels. The problem is that the diameter of the tires is not an absolute constant. Worn tires get smaller, turning faster to cover the same distance, which makes the speedometer mark a higher speed than the real one. Changes in pressure or switching to different sized wheels also affect the reading.
To avoid legal problems, automakers adopt a safety margin. International standards, such as those of the European Union (UNECE), prohibit the speedometer from showing a lower speed than the actual speed. If you are going at 100 km/h, the dashboard will never be able to mark 95 km/h, as this would induce the driver to commit infractions or accidents. On the other hand, the legislation allows it to dial for more (usually up to 10% + 4 km/h).
So in most cases your car is, in fact, slower than your dashboard tells you to. GPS wins in accuracy on straights and constant speeds, but it can suffer “lags” (delays) in sudden accelerations, sharp curves or tunnels, moments when the mechanical speedometer reacts faster.
