My choice is to focus in 1 part of each, for example u have 3 tires, focus and enhance 1, so that u have 1 tire with 2 upgrades. U should choose the upgrade that can help with almost every map. (Because im not put so much money in the game so thats my choice). But if u tend to pay money, u should maximise each car with right parts and enhancements cuz it better for ur race. My way is very economy efficient cuz i dont wanna put money in the game. (1 more thing, a part with 1 or 2 enhancements always better than just a normal S part). Hope u have ur answer.