A hairspring keeps the speedcup steady. The pointer on the speedcup comes to rest where the hairspring balances the force of the revolving magnet. When the vehicle speeds up, the magnet increases its pull on the speedcup, and the speedometer registers a higher speed. When the vehicle stops, the hairspring pulls the pointer back to zero. A device called an odometer registers the total distance traveled by a vehicle.
Many speedometers have a trip odometer that can be reset to zero at the beginning of a trip.
Thus, electronic odometers convert the number of pulses from the speed-sending unit into the total distance traveled. William H. Haverdink, M.S.E.M., Staff Research Engineer, General Motors Research Laboratories. How to cite this article.