Many tires have tread wear indicator bars molded into the tread. When the tread is worn down to where you can see a solid bar of rubber across the width of the tread, it is time to replace the tire. Check your tread depth every 3,000 miles. To save money, also check to see if your current tire is under warranty before making a purchase.
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All the information you need appears on the side of your tire. The basic tire size looks like this: P215/65R15 98H. Depending on what type of vehicle you drive, you may see an LT (Light Truck) instead of a P (Passenger).
Find Out How To Read Your Tire.