 To prevent special preparation or modifications to the participating automobiles for the run, the United States Auto Club purchased the cars at dealerships, checked them and, if certified as "stock", their hoods and chassis were sealed.The factory gas tank was disconnected so fuel use could be accurately measured by using a special tank mounted in the trunk.Because of the many types of automobiles, the Mobil Economy Run had eight classes based on wheelbase, engine and body size, as well as price.