Another question would also be what’s the difference between wheel speed and vehicle speed?
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Early cars used a small gear that ran off the transmission output shaft. A steel cable in a sleeve plugged into the gear and was run to the back of the speedometer. The speedometer has magnetic witchcraft that moves the needle. The gear on the transmission could be changed to adjust for different speedometers, wheel sizes, and final dri e ratios.
In the 80s, manufacturers replaced the mechanical steel cable with a hall effect sensor, usually mounted in the same spot where the mechanical one went. It outputs a voltage or pulses as it spins. This was wired into the computer and the computer then output a voltage to the speedometer. Adjustments for wheels and ratios were programmed into the computer.
When 4 wheel ABS came along, each wheel got a hall effect sensor, where the computer could monitor each wheels speed and correct for loss of traction as well as run the speedometer. The same sensors are used for traction control and vehicle stability control systems.
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