How does Speedometer work?

The Speedometer Cable, powered by the Driveshaft, turns as well. 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). As the speed cup turns, it turns the pointer up the dial, indicating the Vehicle's speed.

The Odometer shows the distance your vehicle has traveled, while the Speedometer shows how fast your vehicle is going.

When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup (called a speed up) attached to the shaft of the pointer on the analog speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, which themselves produce another magnetic field. The effect is that the magnet exerts a torque on the cup, "dragging" it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup are balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tail shaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.

One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction – in this scenario the needle would be driven against its mechanical stop pin on the zero position.

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