Explainer #4
How does a speedometer work?
In a classic speedometer, a cable from the gearbox spins a magnet inside a thin aluminium cup. The moving magnet sets up eddy currents in the aluminium, and they drag the cup along in the same direction. A small hairspring pulls the cup back. The faster the magnet spins, the harder the drag, so the cup and the needle on it turn further before the spring balances them. Modern cars count pulses from a sensor at the wheel instead.
The principle
A magnet can pull on metal that is not magnetic. Aluminium is not attracted to a magnet, but when a magnet moves past it, it creates swirling electric currents in the metal, called eddy currents. These currents make their own magnetic field, which pushes back against the change. The result is a drag force that tries to make the aluminium follow the magnet.
In the speedometer, the magnet spins with the wheels (through a cable from the gearbox), and the aluminium cup sits around it without touching. The cup is attached to the needle and held back by a hairspring. The drag grows in proportion to how fast the magnet spins, and the spring’s pull grows with how far it is wound. The needle stops where the two are equal, so its angle is a direct measure of speed. When you slow down, the drag drops and the spring pulls the needle back.
Modern cars do it electronically. A toothed ring turns with the wheel, and a sensor next to it gives one pulse each time a tooth passes. The car’s computer counts the pulses per second, multiplies by the wheel’s circumference and drives the needle or the display. The same sensors are used by the ABS brakes.
Common myth
The speedometer shows your exact speed
It is not supposed to. The rules used in Europe (UN Regulation No. 39) say a speedometer must never show less than your real speed, but it may show more. When a car model is approved, the reading may be at most 10 % plus 4 km/h too high, so at a true 80 km/h it may show anything from 80 to 92 km/h. Cars checked from the production line are allowed a little more: 10 % plus 6 km/h. Manufacturers therefore set speedometers to read slightly high, often by a few km/h, also because worn or different-sized tyres change the reading. A GPS app usually shows a slightly lower speed than the dashboard, and the GPS is normally the closer one.How this video was made: Rendered 3D model of a simplified eddy-current speedometer and a wheel-speed sensor. The parts are not to scale.
