Tread Depth and Other Telling Signs
In some cases, knowing when to replace your tires is straightforward. For instance, your tires have tread wear indicators built into them, (typically marked by the letters "TWI" and a small arrow on the side of the tire). If these indicators become flush with the tire tread, your tire has less than the required one-sixteenth of an inch of tread remaining, and it's time to replace the tire [source: NHTSA].
You can also check your tire's tread depth using a tread-depth gauge or even a penny (or a quarter, see sidebar). When using a penny, place the penny in the tread's grooves with Lincoln's head upside down. If the tread covers part of his head, you still have enough tread to drive safely. Of course, your tread doesn't always wear evenly, particularly if your tires are misaligned, improperly inflated or otherwise out of balance. Accordingly, make sure to test your tread depth in several places, checking for bald patches as you go, and inspect both the center and outer edges of each tire to make sure the tread is wearing evenly.
In addition to checking tread depth, you should also look for cracks or cuts in your tire's sidewall, particularly in older tires that haven't seen a lot of miles. If you notice a bulge in your tire's sidewall, be particularly wary; your tire has likely developed a weak spot and need to be replaced as quickly as possible.
But are there cases where a visual inspection won't tell you if something's wrong with your tires? And what can you do to prevent tire problems from developing in the first place? Read on to find out.