The United States Auto Club (USAC) is one of the sanctioning bodies of auto racing in the United States. From 1956 to 1979, the USAC sanctioned the United States National Championship, and from 1956 to 1997 the organization sanctioned the Indianapolis 500. Today, the USAC serves as the sanctioning body for a number of racing series, including the Silver Crown Series, National Sprint Car Series, National Midget Series, Ford Focus Series, .25 Midget Series, and Traxxas TORC Series. - Source: Wikipedia