Speedometer and Odometer

The analog speedometer used on cars indicates the speed of the car and records the distance the car has traveled. A speedometer is driven by a flexible cable connected to the speedometer pinion in the transmission. Speedometers are calibrated in miles per hour and/or in kilometers. The instrument also records the distance traveled, recorded in miles or kilometers. That portion of the instrument is known as the odometer. Most odometers record the total distance traveled. Some also record the distance of individual trips. These can be reset to zero. The speedometer and odometer are driven by a cable in a casing. The cable is connected to a gear at the transmission. This gear is designed for a specific model, tire size, and rear axle ratio. The speed indication of an analog speedometer works on the magnetic principle. It includes a revolving permanent magnet driven by the cable connected to the transmission. The magnet sets up a rotating magnetic field which exerts a pull on the speed cup, making it revolve in the same direction. The movement of the speed cup is slowed and held steady by a hairspring attached to the spindle of the speed cup. The speed cup comes to rest where the magnetic drag is just balanced by the retarding force created by the hairspring. The hairspring also pulls the pointer of the instrument back to zero when the magnet stops rotating. As the speed of the magnet increases, due to movement of the car, the magnet drag on the speed cup increases and pulls the speed cup further around. In that way, a faster speed is indicated by the pointer on the face of the dial. The magnetic field is constant, and the amount of movement of the speed cup is always proportional to the speed at which the magnet is being rotated.