The number 51 appears on the Fabulous Hudson Hornet in Pixar's Cars.
Doc Hudson raced as the Fabulous Hudson Hornet, carrying number 51. In the film's backstory, he won three Piston Cups before a crash ended his competitive career and left him bitter about being forgotten by the racing world.
The character is based on the real Hudson Hornet, a full-size automobile produced by the Hudson Motor Car Company. Real Hudson Hornets became famous in early NASCAR, especially during the early 1950s, when drivers such as Herb Thomas achieved major victories.
The number 51 is sometimes confused with Lightning McQueen's number 95. Lightning uses 95 as a tribute to Pixar's 1995 debut feature, Toy Story. Doc's number instead connects him to the Fabulous Hudson Hornet identity and the history of the Hudson marque.