Cruz Ramirez uses racing number 51 after becoming a racer in Pixar's Cars 3.
Cruz begins the film as a trainer, but she takes Lightning McQueen's place in the Florida 500 after he realizes that her talent should have a chance in competition. Her yellow race-car design then carries number 51 as she races against the modern Piston Cup field.
The number connects Cruz to Doc Hudson, Lightning's former mentor. Doc raced with number 51, so Cruz's use of it reflects the legacy she inherits through Lightning's training and Doc's influence. It is not Lightning's number: Lightning remains number 95.
The Florida 500 establishes Cruz as a new competitor rather than simply a replacement body for Lightning. Her victory over Jackson Storm gives Cars 3 a generational handoff, with Lightning moving into a mentor role while Cruz begins her own racing career.