How Speedometers Work
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Eddy Current Speedometers
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Eddy current speedometers were pretty much universal until the 1980s, and are still used on most motorcycles. An eddy current speedometer uses a long, flexible wire cable that runs from the wheels to just behind the speedometer display dial. The speedometer is attached to a magnet, which rotates inside a metal cup but doesn't actually touch it. The metal cup is attached to a spring and to the dial which shows the speed of the car.
Showing the Speed
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As the wheels spin, they turn the cable, which makes the magnet spin inside the cup. This creates a moving magnetic field that flows through the cup. The magnetic field produces an electric current in the cup, which creates its own magnetic field. The magnetic field in the cup is always slightly behind the field in the magnet, so it tries to turn the cup to catch up. The faster the wheels go, the more the magnetic field pulls on the cup. The spring stops the cup from spinning, but does allow it to rotate a bit. As the car goes faster, the cup rotates further. This pulls the needle to the right, showing the speed the car is going at on the speedometer dial.
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Digital Speedometers
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On most cars today, mechanical speedometers have been replaced by digital ones. With digital speedometers, the sensor is right on the car's driveshaft. The driveshaft has a little magnet attached to it, positioned near a magnetic sensor. Every time the driveshaft spins past the sensor, it creates a small pulse of electric current in the sensor. A simple computer counts the number of pulses per second and uses that to determine how fast the car is going. It sends that information to the instrument dial, which displays the car's speed.
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