From a description of Hot Wheels World Race:

There are five teams of seven racers and cars to unlock as you play and each team has only one car available at the start of the game. You unlock new cars by placing third or better (out of six) and collecting 50 out of 100 gold rings scattered around the track. Collecting all 100 gold rings unlocks another racer, although this is tough to do in league mode and is much better achieved in quick races. A track is available once you've got past it in league mode; even if you place last as long as you can continue in the league it will be unlocked.