A system and method of automatically calibrating a vehicle speed sensor signal to provide an accurate speed signal for use in vehicle systems such as a navigation system. The navigation system receives GPS position information signals which are processed to determine current position. A compass provides an indication of the vehicle heading. The navigation system receives the vehicle speed sensor signal and counts the number of signal pulses during a time period. The system determines a pulse ratio of the vehicle speed sensor as a function of a GPS determined speed and the pulse count. The pulse ratio is subsequently used to calibrate the vehicle speed sensor signal. With the navigation system, the calibrated vehicle speed signal is used in connection with a dead reckoning technique for determining the position of the vehicle when GPS signals are unavailable.