How do car speedometers work?


Speedometers are designed for your safety. If you have one and you're probably curious about how the speed gets measured, this article completely simplifies how speedometers work. Understanding how it operates will help you avoid common problems with speedometers!

Car speedometers are built for safety. They help you regulate your speed by displaying the pace at which your car is running. If you’re wondering how accurate speedometers are or you’re interested in learning how the speedometer works, this is the right article to read, provided by!

For easy understanding, we’ve numbered the procedures:

1. The Driveshaft Connection

The driveshaft is a component present in the car used for passing on torque and rotation to the wheels. While driving along a highway, the driveshaft and transmission are spinning at a speed that correlates with the vehicle’s speed. There are a set of gears that connect the driveshaft to the speedometer, then the permanent magnet placed at the periphery of the drive cable is also rotating.

As soon as the engine is powered, the driveshaft begins to rotate, then the cable connected between the speedometer and the shaft follows suit.

2. The Magnet begins to spin

As mentioned earlier, once the speedometer cable starts rotating, the permanent magnet at the end of the dive cable begins to rotate as well - understanding the mechanism of the magnet is essential.

This magnet rotates at the same speed as the driveshaft and the speedometer and it rotates in the same direction with no variations.

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3. The Speedcup

Once the magnet starts to spin, a rotating magnetic field is created, producing forces that affect the speedcup. When the magnet rotates, the force produced causes electrical current to be directed in the cup in little rotating eddies, known as eddy currents. How this force is created can only be explained by the law of electromagnetism.

As the electromagnetic force is created, the eddy currents produces a drag torque that has an effect on the speedcup. Furthermore, the eddy currents lead to anti-clockwise rotation of the speed and synchronize with the speed of the magnet, but only as far as the hairspring allows it.


To display the accurate information for you, a whole complex system works closely behind

4. The role of the Hairspring

The hairspring is a thin coil of wire that regulates the movement of the speed cup. It tries to bring the speedcup to rest, stop it from moving in synchronization with the magnet. As the speed cup turns in a limited speed, the hairspring pulls the pointer up the dial to display the car’s current moving speed.

5. When you increase or decrease the speed

When you set your foot on the accelerator to make the car travel faster, the driveshaft cable turns. When the driveshaft cable turns, the speedometer cable follows suit, which in turn, sets motion to the permanent magnet. Then, the magnet creates a stronger magnetic field, larger eddy currents and a higher deflection of the speedometer’s pointer.

If you’re not driving so fast, the magnet inside the speedcup rotates more slowly, which reduces the strength of the magnetic field, resulting in lesser eddy currents and less deflection of the pointer. As soon as you halt, the hairspring holds the pointer at zero.

There you have it, this is how you get to know what speed you are going at!

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