In Pixar's Cars, Doc Hudson used the racing number 51 during his championship career. His blue Fabulous Hudson Hornet carries the number as a reminder of his earlier life as a celebrated Piston Cup racer.
Doc was a real racing legend before settling in Radiator Springs. He won three Piston Cup championships in 1951, 1952, and 1953, but a serious crash ended his competitive career before he was ready to retire.
After the crash, Doc discovered that the racing world had moved on without him. He eventually became the town's doctor and judge, keeping his racing history private. Lightning McQueen's arrival forces Doc to confront that past and eventually share his experience as a mentor.
The number 51 is linked to the Fabulous Hudson Hornet, a real-life NASCAR-associated car whose racing success helped inspire the character. It is distinct from Lightning McQueen's 95 and Chick Hicks's 86, two other prominent numbers in the film.