This is what worked for me.
Achievement: Unstoppable
credit goes to:Drake O'Phile
source:
http://www.xbox360achievements.org/forum/showthread.php?t=27...Start the Circuit Race at Reliant Stadium (Houston) and get into the lead as fast as you can. Now, hit every jump you see and hold forward on LS so you always land on your front wheels with a loud "clang!". Continue to hit every large jump and land this way, your wheels should start to wobble. If by the end of the second lap you haven't lost a tire yet as you enter the "tunnel" near the finish line start smashing yourself into the wall to finish off your tire. Once it's off the finish line isn't too far away, so simply cross it and get the achievement.