Buying Car Tires: When, Where, and How

What should I know about my tires?

There is a ton to learn and know about tires, and many specialists spend a significant amount of time acquiring knowledge and keeping up with new advances in the industry. However, there are some key points to know that will help you feel more confident when it comes to purchasing tires.

The place to start is on the tire placard for your vehicle. This is usually located either on the driver’s door jam or the fuel door. This placard tells you the requirements for the tires on your vehicle. It will tell you the size of the tire for all positions, the air inflation pressure required, plus the load rating and speed rating your tires need to have.

Size of tire

When you start shopping for tires, first know the size. Today’s tires have a size indicated first by tread width in millimeters—this is three digits. Then there is the “aspect ratio,” which indicates the size of the tire's sidewall. This is a two-digit number that indicates percentage of the tread width. The final number is the rim size, in inches. So, for example, the size P215-60 R16 indicates that the tread width is 215 millimeters, the sidewall of the tire measures 60 percent of 215 millimeters (or 129 millimeters), and the rim opening is 16 inches.

For the vast majority of tire purchases, you will be maintaining the identical size of your tire when purchasing a new set. Sure, when adding custom wheels or modifying your vehicle in other ways, you may consider changing your tire size. But for the purposes of this discussion, you should maintain your tire size. Changing even one of the numbers in your tire size can cause serious issues. Some of your vehicle's electronic components (e.g. the anti-lock brake system) rely on the size of the tire for accurate system functioning. Further, if you benefit in one aspect by successfully changing your tire size, you may sacrifice something else. For example, by changing a tire size, you may improve the way your car handles, but you may also make the ride uncomfortable and harsh.

Uniform Tire Quality Grading ratings (UTQG)

The treadwear is a number that each tire manufacturer uses to rate their tires for longevity of wear. Each manufacturer is required to show a rating, but the ratings are not very strictly regulated. For example, one Michelin tire may rate 300 treadwear and another may rate 600. From this, you can ascertain that the tire with 600 treadwear should last at least twice as long as the 300 treadwear.

But, there is a problem. Since tire manufacturers do not have to show a maximum treadwear, they can legally understate a treadwear rating for the purpose of posturing various models for marketing. Besides that, you cannot compare the number across brands, such as comparing the Michelin 300 rating with a Goodyear 3for treadwear. Therefore, while knowing the treadwear value is a good thing, it is best only to use that number as a minor factor when deciding what tires to purchase.

The traction of a tire is rated AA, A, B, or C for the expected ability of the tire to cope with wet weather. Keep in mind that this means water—not ice and snow. This rating is assigned by testing the tire under controlled conditions as mandated by the United States government. A tire rated AA has the best possible traction. Most tires have AA or A. The B rating is less common and is usually found on economical tire lines.

The temperature is rated A, B, or C for the ability of the tire to dissipate heat. A tire that keeps cool lasts longer and is less likely to exhibit failure under extreme conditions. Look for an A or B here.

Tire speed rating and load rating
A third important designation on the tire is the speed rating (also referred to as a “handling rating”). Nearly all tires today have a speed rating—from S to Y.

If you take the basic interpretation of the speed rating “S,” for example, the tires should be capable of performing up to 112 miles per hour. If you have an economy car, you may wonder—do I really need a tire rated to 112 miles per hour? The answer is yes—if the vehicle manufacturer lists it as a requirement. This is because the higher the speed rating, the better is the tire will handle. They progressively get far more stiff and firm on the sidewalls up to the (Y) rating.

The following are common speed ratings on tires:

S = 112 mph
T = 118 mph
H = 130 mph
V = 149 mph
Z = beyond 149 mph
W = 168 mph
Y = 186 mph

There are two things you should know about speed ratings. First, your new tires should be rated to match the car's required rating, located on a sticker, usually located in the driver’s door jam. Second, all four tires on the car must match in speed rating. For example, you should not mix an “S” with a “V.” Having tires with mismatched speed ratings can cause handling issues and unpredictable steering response.

Finally, the “load rating” of the tire is something you should be aware of, especially for trucks. All tires have a load rating, usually listed as the number before the letter of the speed rating (for example, a 89S would be a load rating of 89 and a speed rating of S). Truck tires, however, also use a letter rating system (C, D, E, F, G, and so on). You must use the correct load rating for your vehicle, as denoted on your vehicle's tire placard.

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