WHEN SHOULD I REPLACE MY TIRES?
According to most states' laws, tires are legally worn out when they have worn down to 2/32" of remaining tread depth. To help warn drivers that their tires have reached that point, tires sold in North America are required to have molded indicators called "wear bars" across their tread pattern from their outside shoulder to inside shoulder. Wear bars are designed to visually connect the elements of the tire's tread pattern and warn drivers when their tires no longer meet minimum tread depth requirements.
However, as a tire wears it is important to realize that while its dry traction and handling will improve…its ability to perform in rain and snow will diminish. At 2/32" of remaining tread depth, resistance to hydroplaning in the rain at highway speeds has been significantly reduced and traction in heavy snow has been virtually eliminated. If rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow it to escape through the tire's grooves. If the water can't escape fast enough your vehicle's tires will be forced to hydroplane (actually float) on top of the water, losing traction.
If snow covered roads are a concern, you should consider replacing your tires when they reach approximately 6/32" of remaining tread depth to maintain good mobility. The reason that you need more tread depth in snow is because your tires need to compress the snow in their grooves and release it as they roll. If there isn't enough tread depth, the "bites" of snow your tires can take on each revolution will be so small that your traction will be reduced. Because tread depth is an important element for snow traction, winter tires start with deeper tread depths than standard all-season or summer tires. Some winter tires even have a series of wear bars molded in their tread pattern indicating approximately 6/32" remaining tread depth.
Since tires affect the personality and performance of your vehicle, all four tires should be as identical as possible or handling problems may arise. If your tires don't match, it is possible that one end of your vehicle won't respond as quickly or completely as the other, making it more difficult to control.
If your tires have a lot of remaining tread depth, but you need to replace just one that has been damaged by an accident, road hazard or a vandal, you should replace it with a tire that exactly matches the others. Select a replacement tire of the same brand, line, size and speed rating. While there may be a less expensive tire available, it wouldn't be a bargain this time because it would be different than the other three tires on your vehicle.
If two of your tires have a lot of remaining tread depth, but you need to replace the other two because they were damaged or have worn out, you should replace them with a pair of tires that come as close as possible to matching your existing tires. While identical new tires are desirable, others of the same size and type can also provide good results. Only consider selecting new tires that are from the same tire category as your existing tires. New tires should be installed on the rear axle. While your vehicle is being serviced ask your mechanic why one pair of tires have worn faster than the others.
Was it caused by a lack of tire rotation, out-of-spec wheel alignment or loose mechanical parts? Once the problem has been found, it can be corrected before it damages your new tires. Keep in mind that your ultimate goal is that all of your tires always wear out at the same time so they can be replaced as a set.
If all of your tires are wearing out together, you have the greatest flexibility in tire selection. If you were happy with the original tires, simply replace them. If you want longer treadwear, a smoother ride or more handling, there are probably tires that will help you accomplish that.
A tire's first requirement is that it must be able to carry the weight of your vehicle. No matter how good a tire you select, if its capabilities are "overworked" just carrying the load, it will have little reserve capacity to help your vehicle respond to quick emergency. So when you are in the selection process, make certain that your new tire's size is designed to carry the weight of your vehicle! Don't undersize. The other size consideration is overall tire diameter. Since many of the functions of today's vehicles are highly computerized, maintaining accurate speed data going into the computer assures accurate instructions coming out. And an important part of the speed equation is your tire's overall tire diameter. For cars and vans, staying within a ±3% diameter change is desirable. Pick-ups and sport utility vehicles (SUVs) are usually engineered to handle up to a 15% oversize tire. Most tire dimensions can be calculated. See the section below on "How do I calculate tire dimensions?"
While at first a ±3% diameter increase or reduction in tire diameter may sound very limiting, in most cases it allows approximately a ±3/4" diameter change. Additionally to help with the selection of substitute sizes, a system called "Plus Sizing" was developed. Use Plus Sizing to take into account the diameters of the available tires and the wheels, and then helps select the appropriate tire width that ensures adequate load capacity. Maintaining the tire's overall diameter helps maintain accurate speed data going into the computer.
Example...185/60R14 85H or 185/60HR14
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 in the example above, the width is 185mm or 7.28".
The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60 =111mm or 7.28"x0.60=4.36".
The last number is the diameter of the wheel in inches. To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.
185mm x .60 = 111mm x 2 = 222mm + 355.6mm (14") = 577.6mm or 22.74"
Do you drive your car only in sunshine, or also through rain and snow? Do you drive your light truck on the road, off the road, or are you the one responsible for clearing the land to build the roads? To successfully meet each of these driving conditions requires a different type of tire. Ask yourself these questions to determine which performance category you should choose from: If you use more than one set of tires and wheels (for example, summer tires in summer and snow tires in winter), you can select tires that exactly meet your diverse needs. If you use one set of tires for every season you may get good performance under many conditions, but you will compromise your vehicle's performance when the conditions are at their worst. So the important thing to do is to select your tires so that they match the worst driving condition you expect to encounter. When you're stuck in the snow or in the mud because your tires don't have the appropriate capabilities, you'll curse their limited performance in your worst driving condition...and you'll quickly forget how smooth and quiet they were at other times! If you only drive around your neighborhood and a "long trip"is one that's just down to the corner convenience mart, almost any tire will do. But if you drive your vehicle on congested city streets and expressways during rush hour you will be better served by more responsive tires.
If you drive extensively on the interstates you will want quiet, smooth riding, long wearing tires. Or if you like to drive quickly on twisting roads or through the mountains you will want good handling tires. And if you drive on the track or in autocross events, you will want the best competition tires available. If your worst driving conditions and your typical conditions are similar, one set of tires will be all you need. If you live at the edge of the snowbelt and infrequently get snow you may want to select an all-season tire. If your SUV is used as the family's station wagon and driven on the road all of the time, overly aggressive light truck tires aren't for you (unless you really like the "look").
If your worst driving condition occurs frequently (you drive through snow all winter) and is dissimilar to your typical driving condition (you commute to work on the expressway during the week and spend your weekends at the beach), you may want to consider selectingtwo sets of tires for your vehicle. Each set will be designed to master the specific conditions without compromising your driving satisfaction at the extremes. While purchasing two sets of tires may appear expensive, the set you're not using won't wear while you are using the other set, and combined they'll provide longer total wear than either set could individually!
HOW IS THE TIRE'S SPEED CAPABILITY ACHIEVED?
As the speed of a tire increases, its components tend to grow or deform. This deformation leads to heat generation. This is a normal effect of the deformation of the tire's components due to centrifugal force. The faster the tire spins, the greater its tendency to deform. And the more it deforms, the more heat is generated.
But for tires driven at high speeds, controlling heat levels is critical. Engineers must find ways to control tire growth so that heat levels are confined to acceptable tolerances.
To overcome this problem, speed-rated tires are designed with advanced belt configurations engineered to resist and control deformation. This allows the tire to be driven at higher speeds without excessive heat buildup.
How does the speed rating relate to handling? The fact that a tire is speed-rated indicates very little about its handling capabilities. The speed rating, as mentioned earlier, is almost entirely dependent on the tire's belt structure.
Think of the speed rating as being like a temperature grade. It tells nothing about the tire's construction, its handling or its wearability. It is merely a measure of the tire's integrity at high speeds.