The Vehicle Speed Sensor (VSS) is typically mounted on the transmission or the differential housing. It measures the rotational speed of the internal gears and sends this frequency signal to the car’s Engine Control Unit (ECU) and the speedometer. This data is crucial not just for showing how fast you are driving, but also for cruise control operation and automatic transmission shift points. A faulty VSS can cause the speedometer to drop to zero, the check engine light to turn on, and the car to shift gears erratically.