Beat the game and and load your cleared save game file to start a new game with all your data from the previous game. Handling is better in the continued game.
When you become the leader of your team, challenge another team. Make all your teammates the lowest ranking drivers. Once they all lose, you will get a chance to race and win up to $10,000 per race. There are five races, allowing you to win up to $50,000 per team race.