In Cars 3, Cruz Ramirez's racing number is 51.
Cruz begins the film as a high-performance trainer at the Rust-eze Racing Center, where she uses modern technology to coach Lightning McQueen. Although she dreams of racing, she initially lacks the confidence to pursue that goal because she believes she does not fit the image of a traditional champion.
McQueen's training journey eventually leads him and Cruz to the Thomasville Speedway, where she helps him rediscover his competitive spirit. Their roles later change: Cruz takes the opportunity to race for the Rust-eze team and becomes a major competitor in the film's climax.
The number 51 connects Cruz with Doc Hudson, whose own racing number was 51. Lightning McQueen's familiar number is 95, while 43 is associated with veteran racer Cal Weathers. Those number links make Cruz's choice significant rather than a random decoration.