1996 Ford Aerostar Owner's Manual - Page 88

Speedometer The speedometer tells you how many miles (kilometers) per hour your vehicle is moving. Your vehicle contains a speedometer which receives its input from the Anti-Lock brake sensor. The speedometer is also used to provide a speed signal for correct operation of the vehicle's Powertrain Control Module (PCM), electronic transmission, and speed control (if so equipped). Because of this, it is very IMPORTANT to change the speedometer's internal conversion constant if the size of the tires on the vehicle is changed. Changing the conversion constant to the proper value when the tires are changed will also ensure that the speedometer retains the original factory-set accuracy. NOTE: The conversion constant can only be changed six times once the vehicle leaves the factory. Your Ford dealership service personnel can change the conversion constant. Some vehicles are equipped with a vehicle speed (mph/km/h) limiting device that is contained within the Powertrain Control Module (PCM). The purpose of this device is to maintain peak vehicle speed below a specified limit. (Vehicle speed limits may vary based on engine displacement.) 87