My car is using the rotation of the front wheel axle to infer the speed. So it depends on tire diameter (tire type and wear), air pressure, temperature, and the

My car is using the rotation of the front wheel axle to infer the speed. So it depends on tire diameter (tire type and wear), air pressure, temperature, and the precision of the mechanics involved, and it's generally nonlinear (difference gets larger at higher speeds). My phone is sampling GPS position at a few Hz, and while the position itself may not be absolutely precise, the relative position from one sample to the next is very good.