First you go and buy the Chevy ss then go to upgrades and go to the engine section and buy the highest one I do not remember what it is called and then click LT on the controler then X pretty much at the same time just click LT first then X and it should sell the engine and you should get the money but you will still own the engine

On "Buy Upgrades" choose an upgrade with 2 or more options above standard (doesnt work with tires).Buy the upgrade that is second to last, and then equip the standard of the option. Then move over the bought upgrade, and pull the right trigger then press "X" in quick sucsetion in that order. You should then recive the value of the uprade at the top of the options list.
NOTE: I find the best car to use is a Lamborghini Gallardo "Turbo" OR the Ferrari F50 "turbo" uprades.

If you need any further expalnation e-mail me.

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 doi it a whole bunch of times and you will get plenty of money. I did I and now I have 120,000,000 credits!