If the factory warranty is 100,000 miles/5 year then yes you are covered to 100,000 miles unless the 5 years comes first. Now, from what was explained to me from my dealership was that it is important to have the timing belt inspected by them periodically (after midway point of your warranty) and this will avoid GM from trying to get out of the warranty repairs if the belt snaps. It seems weird that you are covered during that warranty period but GM will most likely say it was due to your negligence that you didn't get it inspected.......the whole mess could have been avoided if it was inspected by the dealer....blah, blah, blah.
