Get all the unlock for cars.
You'll need to reach 10.000 xp to get this achievement.
You gain 200 xp by finishing first in an online race ( nothing for the second, third, and fourth).
150 xp for a gold medal in single player, 125 for silver and 100 for bronze.
You can do the single player challenges again and again as they'll always grant you the same amount of xp.
The challenge n°5 in the speed ones can be done in less than 27 seconds, it's the fastest one and it can grant you 300 xp per minute.
Be careful, i was at 8.500 xp and 18 win in online races when i downloaded the DLC for the game and it set me back at 0xp and 0 online win. ( the challenges were still done though)