Another question would also be what’s the difference between wheel speed and vehicle speed?
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There’s a electronic sensor or gear connected to a cable at the output side of the gearbox. The cable type uses the rotations to push the needle mechanically, the electronic type just uses a computer. Either way, the number of rotations converts to speed using the wheel diameter.
This gets interesting if you change out your tyres or wheels for a different size – unless you remember to re-calibrate the speedo it’s going to read the wrong speed. Even letting your tyre pressure get low can change the reading slightly, similarly, going around corners means the wheels on each side aren’t quite moving at the same speed and the point where speed is measures isn’t directly at either wheel so the measured speed will never quite be exactly the same as your actual ground speed
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