Like most, when I buy a new car, I cange the oil, and then save the game, after I save the game, I try out every part that you can tune it with (especially for the engine) after I get the part I try out the car with the upgrade on the test track for accelaration and top speed, and complex string and Laguna Seca for handling. Since I saved the game before I do any of the upgrades, if the upgrade was a waste, load the game back and your car will not have the upgrade, you'll still have the money.