When playing Mini 4WD in android device, you can get get some items for free by reaching a certain level or you must get certain points.
Alternatively, you can get those items with gold or Gems where you can make it by participating in some certain events.
You can take part in offline events on Facebook held by the developer
In such events, you get big rewards by reaching certain levels.
To get gems for free, just level up your character until level 18 to get 120 Gems as reward.
Be sure to log in the game regularly to get 60 Energy, Mixing hydrogen of light, 85 purple Battery and 50 gems
Every bonus here will be given only once.