1: Go to Career Mode or Auction House and buy 5 Porsche 914/6's
2: Race and get all of them up to car level 4 because for each one you get a 10% dicount on pereli tires.
3: DO NOT SELL THE PORSCHE'S AFTER YOU GET THEM TO LEVEL 4 OR THE CHAET WILL NOT WORK!
4: Take any car {but the Chrysler ME Four Twleve Works best} and go to Buy Upgrades.
5: Go to tires and get the best tire compound and when you have to pick a tire brand pick Pereli.
6: Buy it, it will cost 17,100 credits then immediately press the X button and sell it, you will sell it for 21,000 credits gaining 3,900 credtits each time do it a whole bunch of times and you will get plenty of money. I did It and now I have 120,000,000 credits!