While GPS absolute position accuracy is only worst case about 2 meters or less, the relative positioning is quite accurate with high resolution on measurements taken in the same area in rapid succession. So when it thinks you’ve moved 10.23 meters for example, it is a very accurate 10.23 meters
Car speeds are calculated from turns of the wheel; the distance traveled per revolution is the diameter times Pi; the wheel circumference then becomes part of the equation. Tread depth just from new to technically worn out is 11/32 to 2/32 or 9/16″ in diameter change (2X radius change, remember) which is about 2.4% of the tire diameter, furthermore the tire is not round but has a moving flat spot that changes its effective circumference and changes with load, tire pressure and temperature
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