Monit rally computers measure distance and speed by counting the pulses received from a Vehicle Speed Sensor (VSS). Depending on the type of vehicle, there are some options for obtaining a suitable signal.
If no suitable VSS is available, the best option is to install a Monit Inductive Wheel Probe. This is mounted on the wheel hub or near a driveshaft, and works by detecting bolt heads as they rotate past the face of the sensor. Once calibrated, it allows the Monit rally computer to calculate distance travelled. (Note, a mounting bracket is required to suit the relevant position on your car).