Doc Hudson raced as number 51 when he was the Fabulous Hudson Hornet in Pixar's Cars.
Doc's racing career is revealed after Lightning McQueen discovers that the quiet local doctor was once a celebrated champion. The number 51 appears on the Fabulous Hudson Hornet's racing livery and connects the older character to his competitive past.
Doc won three Piston Cups before a crash ended his career and left him feeling forgotten by the racing world. His experience helps him understand both Lightning's talent and the dangers of treating racing only as a path to fame.
The number 51 is different from Lightning McQueen's famous number 95. It is also unrelated to The King's number 43, another prominent racing number in the first film.