Take note of stages that you can complete and replay that offer a guaranteed reward that is required for upgrading your vehicles. You can go to the garage and tap a desired item to view all the stages that you can replay to obtain it. The more upgrades available, the better your car will be, which will allow you to win races and unlock new stages easier. Early in the game, choose a car to focus on and use it to progress the story going and win races. Because upgrade parts can be used on any car, begin stockpiling them for use when you get a very really powerful car. Quickly upgrade it after it is obtained.