In Pixar's Cars 3, Cruz Ramirez uses racing number 51 when she competes for Rust-eze.
The number 51 is strongly associated with Doc Hudson, whose racing career is remembered through the identity of the Fabulous Hudson Hornet. Lightning McQueen helps Cruz embrace that legacy when she takes his place in the climactic race.
Cruz begins the film as a highly trained simulator and fitness instructor at the Rust-eze Racing Center. Although she dreams of racing, she lacks confidence because the racing industry has not treated her as a serious competitor. Her training sessions with Lightning gradually reveal her natural ability.
Using number 51 connects Cruz's breakthrough to Doc's history while also showing that Lightning is passing knowledge and opportunity to a new racer. The moment is therefore both a competition detail and a symbol of mentorship.