You can select which car you'd like to use in your next race. A suitable car is automatically selected for you but you can always choose a different one.
Alternative vehicles that suit the upcoming race will have a green side bar and the word 'FIT' on them.
Is your favorite car out of gas? You can refill it to keep it on the track or receive free fuel when you level up. Want to race someone else's ride? Join a Race Club, club members can rent each others cars. These cars become available from the "Select a Car" screen by going into your "Club Garage".
Before each race you can tune your vehicles for better performance or refill them if they need gas. Don't forget you can also limit break cars to exceed the maximum tuning level, just make sure you have the required coins and parts.