In DiRT 3 for the PS3 and Xbox 360, you unlock new cars using the Rep points which is essentially your money in the game. You get rep when you finish races, and obviously the harder the race the more Rep you get.
To get the most Rep per race, keep playing the game until you unlock the Season Four races, or the World Event races. These races will give you a lot more Rep per race than the previous races, allowing you to unlock cars much faster.
Make sure to play above Casual mode, or else the game will slow you down automatically on turns which can actually hurt you quite a bit. Getting used to Casual will make it tougher to switch to a harder mode, so don’t use Casual as a crutch for very long if you want to get better at the game.
