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.

Off topic but this is a real gripe of mine - analogue speedos that go all the way up to 240 kmph (~150 mph). I am only ever going to do half that speed, so why not just make the dial go to only 120/(80) and give the needle (and me!) twice as much precision!

Vehicle speedometers are factory made to display wrong info. I believe they add about 10% to your actual speed.

The reason is error-margin of speedometers, it's about 10% so they err on the side of caution.