How Speedometers Work?

The job of the speedometer is to indicate the speed of your car in miles per hour, kilometers per hour or both. 

1. When the engine turns over, the driveshaft turns to make the wheels spin round.
2. The speedometer cable, powered by the driveshaft, turns as well.
3. The cable spins a magnet around at the same speed inside the speed cup. The magnet rotates continually in the same direction (in this case, counter-clockwise).
4. The spinning magnet creates eddy currents in the speed cup.
5. The eddy currents make the speed cup rotate counter-clockwise as well in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together in any way—there's air in between them.
6. The hair spring tightens, restraining the speed cup so it can turn only a little way.
7. As the speed cup turns, it turns the pointer up the dial, indicating the car's speed.