Join Date: Oct 2009
Location: Bosnia and Herzegovina
Speedometers (as used in cars) always have a slight overreading. This is due to:
- tyre wear (changes continuosly)
- tyre diameter (due to changes of your wheel size)
- outside conditions (tire pressure changes on the different temperatures, causing different diameters)
- vehicle load (again diameter)
In most countries, there is a UNECE (Regulation 39) followed, which allows 10 % reading above the actual speed (80 km/h - allowed to read 88), but forbids reading lower than actual speed.
GPS is generally more accurate, specially in well covered areas, and as the speed goes up GPS is more accurate, totally oposite of a car speedometer, which is less acurate on higher speeds.
Some car manufacturers even declare the speedometer missreading rate in their specs for the car.
GPS is often used by various test teams to measure the exact speed and it is generally considered to be far more accurate than the speedo.
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