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On pretty much all modern cars, it's from the ABS sensors. There's an algorithm that (in crude terms) "averages" the speed of all 4 wheels and displays that as the speedometer reading. It won't be derived from the GPS because speedometer accuracy is a type approval requirement: (they can over-read by up to 10% + 2.5 MPH, but they are not permitted to under-read at all). The manufacturer has to accept liability and responsibility for the speedometer accuracy up to the point of registration, and he can't do this if he is dependent on (in the case of GPS) the American government - or any organisation outside of his direct control.
 
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