Your vehicles need gas to run and you can 'top up' or refill your cars at any time.
Gas is awarded when you level up, so you can use gas that you have in stock or you can purchase gas with coins.
To refill your cars gas you will get a pop-up when you try to race with a empty gas tank in your car.
◎ Purchasing Gas:
If you don't have enough gas in storage, you can purchase more for 1 coin per 1 gallon.