 This term is used to differentiate such a car from a "race car", a special, custom-built car designed only for racing purposes.The actual degree to which the cars conform to standard model specs has changed over the years and varies from country to country.Today most American stock cars may superficially resemble standard American family sedans, but are in fact purpose-built racing machines built to a strict set of regulations governing the car design ensuring that the chassis, suspension, engine, etc.