In Pixar’s Cars, Doc Hudson’s former Piston Cup racing number is 51.
Doc Hudson once raced as the Fabulous Hudson Hornet and won three Piston Cup championships. His number 51 appears in the blue-and-white racing identity associated with his earlier career. After a serious crash and a period of recovery, Doc returned to racing but was pushed aside when newer cars became popular.
In Radiator Springs, Doc hides much of this history while serving as the town’s doctor and judge. Lightning McQueen eventually learns that the quiet Hudson Hornet was a celebrated champion, which changes McQueen’s understanding of fame and racing.
Doc’s design is based on the real Hudson Hornet, a car produced by the Hudson Motor Car Company. The character’s fictional racing record is separate from real-world NASCAR history, although the film deliberately references the Hudson Hornet’s genuine stock-car reputation.