Completion bonus:
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.
Easy money:
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.