Abstract: A timing circuit receives substantially square wave clock signals and generates sine and cos driving signals therefrom at a predetermined frequency for application to first and second mutually perpendicular Hall effect generators. The output signals from the Hall effect generators are algebraically summed to provide a heading signal, the phase displacement of which with respect to the driving signals, is representaive of the orientation angle between one of the Hall effect generators and magnetic north. A counter is synchronized to begin to count clock pulses at the beginning of each period of the driving signals. A logic circuit is effective to disable the counter when the Count signal therein is representative of the orientation angle. The sine and cos of the Count signal are derived and each is multiplied by a distance signal related to the number of revolutions of a wheel of the vehicle being monitored.