In Pixar's Cars, Doc Hudson's former race car identity is associated with number 51.
Before becoming Radiator Springs' judge and doctor, Doc Hudson was a celebrated racer known as the Fabulous Hudson Hornet. His racing number was 51, displayed with a distinctive blue-and-white paint scheme. The film gradually reveals this history to Lightning McQueen, who initially knows Doc only as a stern local authority.
Doc's racing career ended after a serious crash and a difficult recovery. Although he returned to racing, he felt discarded when the sport moved on to younger stars. That experience explains his reluctance to discuss his past and his eventual mentorship of McQueen.
The number 95 belongs to Lightning McQueen, 86 belongs to Chick Hicks, and 43 is associated with the real-life racing number of Richard Petty's car. Those are common alternatives but not Doc's number.