Tires also have built-in wear indicators: each tread groove will have a four or five spots around the tire that are 2/32 of an inch (the minimum legal tread depth) higher than the rest of the groove. These high spots will create a visible gap in the groove once the tread has worn down to 2/32".
Don't just look at one tread groove, either. Check several, to see if one part of the tire is wearing faster than another. Also, look for "cupping," which is a series of flat spots on the edge of the tread; look at the sidewalls for bulges, which indicate a problem with the tire's internal structure. If you can see wire mesh or threads anywhere, that tire is worse than worn out—it may not be safe to drive on. A tire showing any of these conditions needs to be replaced.
A change in your driving pattern may call for new tires before they wear out, however. A move from the Sun Belt to a northern clime may mean you'll be in the market for snow tires; if you'll be using what was previously an off-road vehicle on pavement, you may find you need tires better-suited to asphalt.
Finally, emotion can even play a part in tire buying: you might just like the way a certain tire makes your vehicle look, or you may be very interested in new tire technologies like run-flats.
