In Pixar's Cars, Doc Hudson's racing livery carries the number 51.
Doc is revealed to be the Fabulous Hudson Hornet, a legendary racer from an earlier era of the fictional Piston Cup. His number 51 refers to his 1951 Hudson Hornet, the model associated with his racing identity. Doc's blue paint scheme and racing number distinguish him from the modern competitors Lightning meets at the start of the story.
The character's history is based partly on the real Hudson Hornet, an automobile that achieved notable success in early stock-car racing. Pixar combined that historical inspiration with an original backstory in which Doc wins three Piston Cups before a crash ends his racing career.
Viewers sometimes confuse Doc's 51 with Lightning McQueen's 95 or The King's 43. Those numbers belong to different racers and reflect different identities within the film's racing world. Doc's number also helps connect his quiet life in Radiator Springs with his forgotten championship past.