In Pixar's Cars 3, Cruz Ramirez uses racing number 51 when she becomes a racer. The number connects her to Doc Hudson, whose racing number was also 51 during his Piston Cup career.
Cruz begins the film as a high-performance trainer at the Rust-eze Racing Center. She helps Lightning McQueen prepare to compete with Jackson Storm, but her own driving ability becomes increasingly important as the story develops.
During the final race, McQueen changes places with Cruz. She enters the competition in his place and ultimately wins, while McQueen adopts a new role supporting her career. The switch makes Cruz's number a visual link between her, McQueen, and Doc's legacy.
Cruz's arc is one of the film's central themes: racing talent is not limited by age, background, or the expectations of a training role. Her victory also gives the series a new leading racer without removing McQueen from the story.