Doc Hudson is painted with the racing number 51 in Pixar’s Cars.
The number connects Doc to his hidden past as the Fabulous Hudson Hornet, a celebrated race car from the early 1950s. In the film, Doc initially presents himself as Radiator Springs’ physician and judge, but Lightning McQueen eventually discovers that he was once a champion racer.
Doc’s design is based on the Hudson Hornet, a real American automobile produced by the Hudson Motor Car Company. The real Hudson Hornet became famous in stock-car racing, especially during NASCAR’s early years. The fictional character’s number and nickname deliberately echo that history.
The number 51 is easy to confuse with Lightning McQueen’s number 95 or The King’s number 43. Those numbers belong to different characters and symbolize separate racing identities. Doc’s former career also explains why he understands racing technique even though he no longer competes.