In Pixar’s Cars, the championship contested by Lightning McQueen and other racers is called the Piston Cup.
The Piston Cup is the film’s fictional stock-car racing competition. Lightning enters the final race tied with veteran racer Strip Weathers, known as The King, and aggressive rival Chick Hicks.
The championship is modeled on the imagery and traditions of American stock-car racing, especially NASCAR. Its sponsor-based teams, pit crews, race commentators, and oval-track competition make it the main sporting framework of the first film.
The World Grand Prix is a different competition introduced in Cars 2. It takes place across several countries and features international racers, so it should not be used as the answer for the original Cars championship.