I purchased a new bike last May, a 2006 Giant TCR3. I've put on about 5K miles since then (I was off the bike for several months with a leg injury). My weight (bike, me, and Camelbak) is about 180 - 220 lbs. Today I noticed that my rear tire was thread worn. I've never worn out a tire before. I usually have to replace them about once a year as they tend to crack. The tires tend to be the less expensive tires. This one was a Michelin. I'm told it's about a 60 - 80 tire. I've replaced the tire but I was a little caught off guard by this. Is this normal?