Speed is determined by the data provided by the phone's GPS.

GPS speed might be less accurate under conditions where the GPS signal is weak or obstructed, such as in tunnels, between tall buildings, or beneath heavy tree cover. In these instances, the speedometer, which operates either mechanically or electronically in conjunction with the car's transmission, may offer a more precise reading of the current speed.

One should anticipate some discrepancies between the speed indicated by the GPS and the car's speedometer based on the conditions.

However, in most instances, the two measurements are very close.