Coins are the in-game currency used to upgrade different in-game parameters such as parts. You also need gold to skip enemies in Quick Fights.

There are currently four ways to get coins:
1. Win Championships
Complete Championship Stages to earn gold, as well as Skill Points, Gems, and a Supply Box.
2. Defend against Championship opponents
If your car is powerful enough, it will earn you some coins even when you are offline!
3. Open Boxes
Earn boxes by completing the quick fights and stages in the championship. Each box contains a certain amount of coins!
4. Sell Parts
If you don't have enough gold, you can sell one of your parts. To do so, drag the part to the bottom right corner of the screen. Please note that this action is irreversible. You will lose the sold part and it won't be possible to recover it.
If you don't have enough gold to upgrade the part, you can spend Gems instead.