Tires exist for every variation of automotive use imaginable. For example, there are tires tailored to everyday passenger car use, racing, off-roading, commercial transport, agricultural vehicles, boat trailers, and much more. While internal construction varies between these applications, the exterior of all tires is a specially blended rubber compound that provides airtight sealing, cushioning from impacts, and firm grip on road surfaces.
That rubber compound is designed to wear gradually under "normal" driving conditions, which means roads are in good enough shape not to cause excessive tire wear or damage. Because tires and tire life can vary greatly depending on construction and individual use, automakers stay away from specifying replacement intervals.
That means it's up to you to keep an eye on the condition of your tires to ensure your safety isn't compromised. According to recent studies by the U.S. National Highway Traffic Safety Administration, approximately 9 percent of automobile accidents are related to tire issues. This is because most motorists make the mistake of not thinking about their tires until it's time to suddenly brake, swerve, or change a flat.
After reading this article, you won't make the mistake of ignoring your tires - because we cover how they wear, become damaged, and when they need replacement.
Modern passenger car tires feature a "steel belted radial" design that was first developed in the 1940s. Known simply as radial tires, these contain a series of radial plies mounted across the tire from left side to right - perpendicular to the direction the tire travels. Compared to older "bias ply" tires that laid the plies out diagonally, radial tires generate less heat, last longer, and provide superior ride quality thanks to more flexible sidewalls.
Note: For more details on tire construction, see our related article The Hidden Secrets of Tires. And if you're interested in learning about what various tire dimensions mean, our article Replacing Your Original Factory Tires 101 explains everything.
The Importance of Tire Tread
Unless you're driving on a smooth, dry racetrack at all times, you'll need grooves in your tires to maintain grip. What these grooves do is effectively channel out rainwater on the road surface that would otherwise cause the tire to surf on top of the water instead of gripping the pavement. And if you drive through dirt, mud, or snow, those grooves also take an effective bite into the slippery stuff to propel you forward. Today's tire manufacturers make a science out of creating tread patterns that are effective at achieving good grip levels on both dry and slushy ground.
Note: For more details about best tread patterns for various uses, see our related article on the subject.
Tires without any grooves at all are known as "racing slicks", because their 100% flat surface creates the most possible adhesion on smooth tracks. However, since racing tires would lose traction at slow speeds with the barest amount of water on the pavement, they are not legal for street use.
Tread Depth Matters
Take a closer look closer at snow tires or off-road tires and you'll notice the tread grooves are deeper and wider. Why? Because those aggressive grooves provide edges that take a harder bite into the soft, slippery stuff. While such tires generate more noise and can even cause a slight degree of swaying during everyday use on paved roads, we use them as an example of how tread depth can make a difference when it comes to real-world traction. Without tread depth, that hard bite becomes nothing more than an ineffective nibble.
Tread depth is one of the most important things to pay attention to when inspecting your tires. Perhaps the easiest way of checking tread depth is to slide any U.S. quarter into one of the center grooves, with George Washington's head upside down. If the top of his head is level with the tread, that means 4/32nds of an inch of tread depth is left. This is enough to get by in moderate rain or snow, but you should start browsing our website for new tires soon. If you deal with higher levels of snow and ice, consider replacing the tires at 5/32nds or more accordingly.
When tire grooves are shallow enough to see space on the quarter above the top of Washington's head, then you're at 3/32nds of an inch - and at greater risk for losing grip and going into a slide. Replace your tires immediately.
Treadwear Ratings On Tires
By law, passenger car tires sold in the U.S. must display a treadwear rating number on their sidewall. Note that this rule is not mandatory for off-road tires, tires designed for trailers, or temporary spares. Uniform Tire Quality Grade Standards (UTQG) were specified to create a uniform rating system for consumer tires when it comes to treadwear, temperature resistance, and traction ability. It's interesting to note that the treadwear test is conducted over 7,200 miles in Texas, where wheel alignments, tire rotations, and pressure adjustments can be performed every 800 miles.
After 7,200 miles, tires are compared to the course monitoring tires - which are the control subject used to set what the comparative wear point will be. Whatever the wear level of the course monitoring tires is, it's given a rating of 100. A tire that would last twice as long as the control tire is rated at 200, and a tire that would last five times as long would be rated 500. Ultimately, it's the tire manufacturers (not the US government) that set the final treadwear rating - within guidelines. For this reason, comparing the ratings of two different brands of tires may not be as exact a comparison as different models of tire from the same brand would be.
Generally, tires made of harder rubber compounds to achieve a lower rolling resistance (for higher fuel efficiency) will score the highest treadwear ratings of around 700 to 800. Conversely, performance tires designed for roadholding and/or racing will have the lowest treadwear ratings (below 100 in extreme cases) because their rubber is made of a stickier, softer compound that gets scrubbed off faster.
Treadwear ratings are generally a good indicator of how rapidly a tire will wear. We say take that into account when you check the depth of your tread. Because if you're at 4/32nds of an inch, a sticky performance tire will take less time to wear down to an unsafe point than a high-efficiency one will.
Tire Age And "Dry Rot"
Age Affects Tires - Whether They're Driven A Lot or A Little
Direct sunlight, UV rays, and ozone in the atmosphere all react with rubber in automotive tires, causing the compounds to dry up, harden, and even crack over time. To reduce this "dry rot" effect, tire manufacturers blend waxes into the rubber. However, a tire must rotate and flex for these waxes to rise to the surface and be effective. For this reason, a rarely-driven vehicle can be even more susceptible to dry rot, not less.
When a tire reaches five years of age, we recommend close examination of the sidewalls - which is where you'll see cracking start to develop first. After ten years of service, the rubber has begun to harden enough that tires should be replaced as a precaution. Both of these recommendations are made regardless of how much or little the tire has been used. Don't forget the spare as well.
The safety issue most likely to occur with older, dry-rotted tires is that steel belt reinforcements within the tire can shift out of place when rubber hardens to the point where it's no longer flexible enough to hold things together. A belt shifting may cause a blowout while driving, or it can create a sudden and horrendous shimmy in that tire.
How To Know The Week and Year Any Tire Was Manufactured
A quick and easy way to find out the precise age of any tire sold in the United States is to look for a 4-digit DOT (Department of Transportation) marking on the sidewall. This 4-digit number refers to the week and year a tire was manufactured. For example, a number of "3507" means that tire was produced during the 35th week of 2007. Note that while many tires will have the date code on the outside-facing sidewall, there are cases where you'll need to look at the inward-facing side to see the day.
Spotting visible signs of damage on your tires is equally (if not more) important as observing treadwear. As you look over your tires for the following signs of trouble, remember to inspect the inboard edge of the sidewalls which often gets overlooked.
Blisters on a tire sidewall are a result of trauma - usually caused by a deep pothole, curb, or hard speed bump hit. After the initial impact squeezes the tire against the rim, the inner lining of the tire is cut. Air then leaks in between inner and outer sidewall areas in a concentrated pocket, forming a visible swelling. Blisters can range in size from a small bulge to a large bubble that looks ready to pop any second.
Any blisters in the sidewall are a blowout just waiting to happen - and happen soon, not in the distant future. Since there's no way to effectively repair a blister in the sidewall, the tire must be replaced immediately. If your spare tire is inflated and in good order, swap it on the vehicle in place of the blistered one.
Damage From Driving An Underinflated Tire
Another thing to be mindful during inspection is damage that may have occurred if a tire was ever driven for any amount of time while severely underinflated. Internally, the sidewall becomes shredded all the way around because of friction caused by sections of the inner sidewall rubbing together when the tire had collapsed upon itself. On the outside, you may notice a ring around the tire where rubber has become visibly shredded. Or, you may see what appears to be a cut line all the way around the outer sidewall.
Regardless of exactly how inner sidewall damage manifests itself on the outside, the tire has become too compromised for continued safe use. Replace it as soon as possible, because continuing to drive on it can result in a blowout.
Unlike nails that have lodged directly in the middle of the tire tread, punctures in or close to the sidewall cannot be safely repaired. Physically, it may be possible to plug such a hole once a nail or screw is removed - but the tire plug will not hold for long due to the fact that the sidewall area flexes too much. You'll need to replace any tire that's been punctured on the sidewall or along the first outer inch of the tread.
We'll also point out that run-flat tires do not hold plugs well because their harder rubber blend does not allow the flexibility required. If you've got a puncture anywhere on a run-flat tire, you'll need to replace it. Tire warranties purchased for run-flat tires will typically cover replacement due to puncture.
Opportune Times To Replace Tires
While we'll grant you that tires are not inexpensive (for some larger SUVs and pickups, a single tire can run over $200), there are times when it's in your best interest to replace them as a pair of two or as a set of four, even if only one tire "failed".
Keep in mind that it's four small patches of rubber which are the total contact between your vehicle and the road. All your control and safety are dependent on those contact patches. As we see it, the only time you should be buying just one tire is when all the tires are relatively new, and one of them has completely failed. Otherwise, at a minimum, tires should be purchased in pairs per axle.
You always want to be certain that all four tires are of the same size and construction (unless you are running staggered tires). It can be "penny wise and pound foolish" to replace only a pair of tires if the two remaining ones are down to 3/32" or less.
As a final comment regarding tire quantities to purchase, be aware that there are some AWD systems which are hyper-sensitive to tire diameter. Running such systems on two or three new tires, and one or two worn ones, can cause the AWD system to malfunction, costing you a repair that is many more times the cost of a set of 4 tires. When it comes to the vehicle's rubber, we suggest that it's better to be safe than sorry.
You are now armed with the knowledge to recognize when a tire is at the end of its lifespan and needs to be replaced. Considering the inconvenience of calling a tow truck or changing a tire on the side of the road after a blowout occurs, delaying replacement of a problem tire is asking for trouble.
If you're not sure which tires would match your car or truck, just enter its year, make, and model details in our "Select Vehicle" box along the top of the screen. Our system will then present only those items which are an exact, vehicle-specific fit. And if you have further questions, we also welcome any product questions you may have by phone seven days a week!