How do i unlock the extra cars?

Well there are some cars that you get by entering a code in the "Under the Hood" section. These unlock sponsor cars which are just copies of other cars in the game except with a skin with a sponsor on it. Depending on where you live you may only be able to get some of these cars. Check the Cheats section for more details and the codes.

There is one car that you can unlock online called the Carbon Uberschall 8. To get it you must complete 2 sets of freeburn challenges. So for example completing all of the 2 and 3 player freeburn challenges counts as 2 complete sets. Timed challenges do not count towards this.

There are 5 other carbon cars that you unlock for completing certain things. Smashing all of the billboards will unlock a carbon car. This is the same for gate smashes, super jumps, time road rules and showtime road rules.