In Pixar's Cars, Doc Hudson used the racing number 51.
Doc was once the Fabulous Hudson Hornet, a celebrated racer who won three Piston Cups before a crash ended his competitive career. His number, 51, is displayed in the historical photographs and memories that reveal his past to Lightning McQueen.
The character is based on the real Hudson Hornet, a car associated with early stock-car racing success. The film uses Doc's hidden identity to connect Radiator Springs with the history of American automobile racing and to show that an older champion can still teach a gifted newcomer.
Doc's number is different from Lightning McQueen's 95 and Chick Hicks's 86. The three numbers are especially easy to mix up because the characters compete in the same Piston Cup storyline, but 51 belongs to Doc's racing career.