A speedometer or speedometer is a gauge used to measure and display the instantaneous speed of a vehicle. Now they are generally installed in cars, they have been available as options since the 1900's, and they have become standard equipment since around 1910. [1] Other vehicles have speedometers with specific names and use other methods of sensing speed. For boats, this is a maintenance area log. For airplanes, this is the airspeed indicator.

Charles Babbage is believed to have created an early type of speedometer, usually suitable for locomotives.

Electric speedometer by Josip belu? i？ invention. In 1888, it was originally called a speedometer.

It was originally patented by Otto Schultze on October 7, 1902, [5] using a rotating flexible cable, which is usually driven by a gear train connected to the vehicle transmission output. Early Volkswagen Beetles and many motorcycles used front wheel drive cables.

When the car or motorcycle is running, the speedometer gear component turns the speedometer cable, and then turns the speedometer mechanism itself. A small permanent magnet fixed on the speedometer cable interacts with a small aluminum cup (called speed cup), which is fixed on the pointer axis of the analog speedometer instrument. When the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, and the eddy current itself produces another magnetic field. As a result, the magnet applies torque to the cup-shaped part in the rotation direction of the cup-shaped part, thus "dragging" its torque, thus dragging the speedometer pointer, and there is no mechanical connection between them.

The pointer shaft is held at zero position by a fine torsion spring. The torque on the cup increases with the rotational speed of the magnet. As a result, the increase in vehicle speed will cause the cup and speedometer pointer to twist towards the spring. The cup and pointer will rotate until the torque of the eddy current on the cup is balanced by the reverse torque of the spring, and then stop. Considering that the torque on the cup is proportional to the speed of the car, the deflection of the spring is proportional to the torque, and the angle of the pointer is also proportional to the speed, the equidistant mark on the dial can be used for the speed difference. At a given speed, the pointer remains stationary and points to the corresponding number on the speedometer dial.

The return spring is calibrated so that the given rotational speed of the cable corresponds to a specific speed indication on the speedometer. Several factors must be considered for this alignment, including the ratio of the tail shaft gear that drives the flexible cable, the final ratio in the differential, and the diameter of the driven tire.

One of the main disadvantages of the eddy current speedometer is that when driving in reverse, it cannot display the speed because the cup will rotate in the opposite direction - in this case, the needle will be driven in the zero position against its mechanical stop pin.