Programming the speedometer
The TCS or ESP control module reads 48 pulses per wheel revolution. Depending on the tire and wheel dimension (physical circumference), one kilometer will correspond to a different number of pulses when the engine control module receives the value from the TCS or ESP control module. It is possible to change how the engine control module interprets the total number of pulses per kilometer so that it corresponds to the current tire dimension on the car. Programming is required so that the speedometer and odometer will provide the driver with the correct information. On many markets, it is a legal requirement that the value shown by the speedometer is between 0 and 10 km/h too high.
If the speedometer is programmed for a certain type of tire and new wheels with larger circumference are fitted, the speedometer will show a speed that is too low in comparison with the one that should be displayed.
The current tire dimension is programmed with Tech 2.