In Pixar's Cars, Doc Hudson carries the racing number 51.
The number refers to the year 1951, when the Hudson Hornet first appeared. In the film, Doc is introduced as the respected judge and doctor of Radiator Springs, but Lightning McQueen eventually discovers that he was once a celebrated race car.
Doc's racing history is based on the real Hudson Hornet, which dominated early stock-car racing. The character's appearance, including his blue paint and rounded body, reflects the design of the Hudson Hornet produced by the Hudson Motor Car Company.
The number 51 is easy to confuse with Lightning McQueen's number 95. Lightning uses 95 as a reference to 1995, the year Pixar released Toy Story, while Doc's 51 points to the Hudson Hornet's debut year. Doc's racing number is therefore both a character detail and a historical reference.