* Drive Gear: A gear, typically located on the output shaft of the transmission, rotates with the driveshaft.
* Speed Sensor (Vehicle Speed Sensor - VSS): This sensor is positioned near the drive gear. It's either magnetic (using a reluctor ring on the gear to generate pulses) or optical (using light to detect the gear's rotation). Each rotation of the gear creates a specific number of pulses.
* Transmission of Pulses: The VSS sends these pulses to the instrument cluster's electronic control unit (ECU).
* Calculation and Display: The ECU counts the pulses and, using the known relationship between pulses and vehicle speed (calibrated during manufacturing), calculates the speed and displays it on the speedometer.
Essentially, the faster the driveshaft rotates (the faster the car is going), the more pulses the VSS sends, resulting in a higher speed reading on the speedometer. If any part of this system – the drive gear, the VSS, the wiring, or the ECU – malfunctions, the speedometer may be inaccurate or not work at all.
