A vehicle speed sensor is often not something people consider when thinking about their cars. They aren't something that a lot of car owners even know about.
Below is a snapshot of what types of vehicle speed sensors there are, and what they do.
Transmission Speed Sensor
This speed sensor indicates the speed at which the car is actually traveling. It is used to control certain handling systems, as well as certain braking systems.
It is controlled by a signal generator that is spun, and makes an electrical pulse that is then sent to the vehicle's computer. Often times, it will be used to control transmission shift points in addition to the systems mentioned above.
ABS Speed Sensor
An ABS speed sensor is present in all cars that contain an ABS system. There are four sensors that are at the wheel locations. The axle shaft will have a notched wheel attached to it that will turn the inside of the wheel sensor. It then gives it a signal that goes to the ABS computer telling the speed of each wheel separately. That information will then in turn engage and even disengage the brakes as necessary to prevent any skidding, etc.