1997 Ford Aerostar Owner's Manual - Page 22
1997 Ford Aerostar Manual
Page 22 highlights
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.) If you are experiencing an "engine cut-out" condition at high speeds, it may be the result of this limiting device. It is a normal condition and can be avoided by reducing vehicle/engine speed. Speedometer 19