Five Of Your Best 15 
Win 5 Pro Tour races over Xbox LIVE.
Can be done in Team races, but you need to finish 1st. So this achievement unlocks after finishing 1st 5 times.
With Great Honor 15
Achieve a 'Cautious' impact rating in Multiplayer races (Pro Tour or Jam Session).
Unlocking this achievement depends on your driving style and your current impact rating. There are 3 ratings. You'll get the red rating when you're an agressive driver. The yellow one is an average rating and the green one is for the driver who almost never crash into other players or the environment. You can get this achievement easily by setting up a couple of races with impact rating on. Now complete these races without crashing or bumping and this achievement will come to you quickly.