Doc Hudson raced with the number 51 in Pixar's Cars.
Doc Hudson is revealed to be the Fabulous Hudson Hornet, a legendary Piston Cup champion. His racing number, 51, appears on the blue Hudson Hornet associated with his earlier career and reflects the number used by the real Fabulous Hudson Hornet.
Doc won three Piston Cup championships in 1951, 1952, and 1953. A crash ended his racing career, and he later became Radiator Springs' physician and judge. His experience allows him to recognize McQueen's talent while warning him about the dangers of arrogance.
Lightning McQueen races with number 95, while The King uses 43. Chick Hicks uses 86. These numbers belong to different racers and are therefore frequent alternatives to Doc's number 51.