When your tires start to go, you really shouldn't. Account for tread wear, exposure to the elements and tire age, and you'll know when it's time to change.
Tread wear
To test your tread depth, Abraham Lincoln's the best man for the job. Put a penny head first into a few tread grooves. If you can see his whole head, it's probably time to tell those tires to hit the road.
What's up with tread depth?
Tire treads, like people, shouldn't be shallow. It's not safe. Good treads give tires a good grip on the road. Worn treads don't respond well in rain and snow, and cause other car parts to wear before they should.
Exposure to the elements
Honestly, your tires can't handle the heat. High temperatures and ultraviolet rays could make your tires go a little early.
Tire age
Tires don't exactly get better with age. Vehicle manufacturers recommend replacing your tires after six years. Tire manufacturers usually say ten. You should check your specific manufacturer's recommendation. Or you can always ask us. We tend to know these types of things.