Racing is a gamble and you need to know when you need to buy a new car since you can't defend all the time on your trustee car. There would come a day when you would need to invest on a better one. Besides, racing would wore out your car and it could retire at an early race, even if you can't seem to notice it.
Drive your car as fast as you can till you cross the finish line for the third time. In this game, you can't proceed to the next challenge if you can't get at least the third spot. You are going to earn money according to your position in the race. Buy better car once you have enough money for it.