Why does a car speedometer show a slightly higher speed than the actual one?
ave you noticed that your car’s speedometer often reads a little higher than GPS? This is usually normal.
Have you noticed that your car’s speedometer often reads a little higher than GPS? This is usually normal. A speedometer is designed to avoid underestimating the vehicle’s actual speed.
The speedometer calculates speed from wheel rotation
Most cars estimate speed by measuring how quickly the wheels rotate. Sensors read the wheel rotation, and the car’s electronics use that information to calculate speed. In other words, the speedometer does not directly measure the car’s movement over the road.
Tyre size and wear can also affect the reading. If a wheel’s rolling circumference changes, it rotates differently at the same actual road speed, which can affect the speedometer. That is why it is important to use the tyre size specified for the vehicle and keep tyres properly inflated.
Why is the speedometer designed to overread?
If a speedometer underestimates speed, a driver may unintentionally exceed the speed limit. UN Economic Commission for Europe Regulation No. 39 therefore requires the indicated speed not to be lower than the vehicle’s actual speed. It allows a certain amount of overreading: the indicated speed may be up to 10% of the actual speed plus 4 km/h higher. This is a type-approval limit, not a claim that every car will show the maximum difference.
For example, at an actual speed of 100 km/h, the regulation’s upper limit is 114 km/h. In practice, many cars show a smaller difference.
Is GPS always more accurate?
GPS calculates speed from changes in position. On a straight road at a steady speed, it can provide a useful comparison, but buildings, tunnels, terrain and a short measurement delay can affect the signal. GPS should not be treated as a perfect measuring instrument in every situation.
If your speedometer appears to be significantly inaccurate, check the tyre size and pressure, and have the instrument checked if needed. When driving, use the car’s speedometer and follow the posted speed limit.
Source: UN Regulation No. 39 in the Official Journal of the European Union
