When the vehicle is running, the speedometer gear assembly turns the speedometer cable, and then the speedometer mechanism itself. The pointer shaft is held in the zero position by a fine torsion spring. At a given speed, the pointer will remain still and point to the appropriate number on the speedometer dial. The return spring is calibrated so that the given speed of the cable corresponds to the specific speed indication on the speedometer. The calibration must take into account several factors, including the drive flex cable, the difference in the ratio of the tail shaft gear in the final drive ratio, and the diameter tire being driven.
Many modern speedometers are electronic. In the design derived from the earlier eddy current model, the rotation sensor installed in the transmission emits a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft and therefore to the speed of the wheels. The sensor is usually a set of one or more magnets mounted on the crown wheel of the output shaft differential or a toothed metal disk located between the magnet and the magnetic field sensor. When the part involved rotates, the magnet or tooth passes under the sensor, generating a pulse in the sensor each time, because they affect the magnetic field strength to be measured. Instead, especially in vehicles with multiple wiring, some manufacturers use pulses from ABS wheel sensors that communicate with the dashboard via the CAN bus. Most modern electronic odometers have additional functions other than the eddy current type, which can display the vehicle's speed when driving in reverse gear.
The computer converts the pulses into speed and then displays the speed on an electronically controlled analog pointer or digital display. The ECU or vehicle control system also uses pulse information for a variety of other purposes, such as triggering ABS or traction control, calculating average travel speed, or adding an odometer to replace the odometer directly rotated by the odometer cable. Another early form of OEM speedometer relied on the interaction between a precision watch mechanism and a mechanical impulse wheel driven by a car's wheel or transmission. The position of the speedometer needle reflects the relative magnitude of the output of the two mechanisms.