Its a function in the ECU. On the Digital input there is a calibration figure. Adjusting this allows for different wheel sizes.
A Digital Input may be connected to an electronic wheel speed or vehicle speed sensor (typically located near the output of the gearbox, or as an output from the vehicle dash board) to enable speed to be calculated. These Digital Inputs can then be used to configure the Driven and Non-Driven Speed Sources.
The Calibration number allows the ECU to calculate speed in km/h.
Note: In some cases erratic speed signals can occur due to unusual speed signal shapes. Changing from Rising Edge to Falling Edge or vice versa may help with this problem. This problem usually presents as random spikes of increased speed.
Set-up Procedure: The Calibration number is the number of pulses received over a 100 m distance.
Method 1: Drive the vehicle at a known speed and adjust the calibration number until the Vehicle Speed Runtime Value displays correct.
Method 2: Wheel Speed Sensors Only. Calibration Number = (Number of Sensor Teeth / Wheel Diameter) * 31.83. Note that wheel diameter is in meters (m).
Typical values for speed calibration numbers range from about 170 to 280. The maximum frequency the digital input can accept is 500 Hz.
Note: Wheel Speeds can be checked in the Runtime Values Digital Inputs tab