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Understanding All About Car Tires

    Vehicle Type

    • There is usually a letter (or letters) at the beginning of a tire size. This letter tells you what kind of car the tire is intended for. If it starts with a P, the tire is intended for a passenger vehicle such as a car, SUV or small pickup truck. If the tire code starts with LT, the tire is made for a light truck, which includes large pickups and vans. The letter T designates a temporary spare; the letters ST mean the tire is intended for a special trailer. If there is no letter in front of the tire size, it is referred to as a "Euro-metric" tire and is typically the equivalent of a tire that is designated P.

    Section Width

    • Directly after the letter on a tire size, there is a three-digit number. Or if it's a Euro-metric tire, the three-digit number will be at the beginning. This number tells you the section width of the tire in millimeters. The section width is measured from the inner sidewall to the outer sidewall. So if your tire is a P205/45R 17, the section width is 205 millimeters.

    Sidewall Aspect Ratio

    • The next part of the tire size is the sidewall aspect ratio. The sidewall aspect ratio is a two-digit number that directly follows the section width. The sidewall aspect ratio represents a percentage of the section width. So if your tire is a P205/45R 17, the sidewall from tread to rim is 45 percent the section width. The higher the number, the taller the tire.

    Internal Construction

    • After the sidewall aspect ratio number, there is often a letter. This letter tells you what the internal construction of the tire is. If that letter is an R, the tire's body plies are radial, and they are positioned as if they are radiating out from the middle of the wheel. If the internal construction letters is a D, the body plies are positioned in a diagonal pattern and crisscross one another. You might also see a B, meaning the tire is belted, but those tires are extremely rare.

    Wheel Size

    • The last number on the tire size identifies the rim size. If your tire is a P205/45R 17, the tire is meant for a 17-inch rim. You may also see rim sizes in expressed in millimeters, but it is easy to tell which unit of measure is being used. It's unlikely your car would have 17mm rims or 360-inch rims.

    Speed Rating

    • The last letter you'll notice on your tire's sidewall is the Speed Rating. These letters range from M-Z and let you know how fast you can safely drive on the tires. It's not necessarily all that important in America, where it is illegal to drive over 70 miles per hour in most states, but in countries such as Germany, with its famous autobahn and the Isle of Man (where many roads don't have speed limits), it is important to have tires that can hold up under high-speed conditions.
      The lowest-rated tires you can buy are M rated. They are approved for usage up to 81 miles per hour. M-rated tires are used on passenger cars, but only as temporary spares. The next step up from an M-rated tire is an N-rated tire. N-rated tires are approved for speeds up to 87 miles per hour. After that are P-rated tires (up to 93 mph), Q-rated tires (up to 99 mph) and R-rated tires (up to 106 mph). N-rated through R-rated tires are only available for light trucks and not passenger vehicles.

    Speed Ratings S-Z

    • The next step up in speed ratings is an S-rated tire, which is approved up to 112 mph. After that are T-rated tires (up to 118 mph), U-rated tires (up to 124 mph) and H-rated tires (up to 130 mph). All of these tires, from S through H, can be found for passenger cars and light trucks. H-rated tires are the most common and are found standard on most passenger cars in America.
      After H-rated tires come V-rated tires. V-rated tires are approved for use up to 149 miles per hour. Z-rated tires are approved for speeds in excess of 149 miles per hour. These tires can only be used on passenger cars. Often times, you will find Z-rated tires for sale at a sharp discount from their lower-rated counterparts. This is because Z-rated tires wear out much faster than the lower-rated tires. While it might be tempting to save some money up front, it's unlikely you'll be able to drive over 150 miles per hour in America, and you'll end up paying for it in the long run when you have to purchase new tires much sooner than you would if you bought lower rated ones.
      Z-rated tires used to be the highest-rated tires available, but with more modern cars being able to reach speeds way above 149 miles per hour, two new speed ratings have been introduced. W-rated tires have been tested safe at speeds up to 168 miles per hour, and Y-rated tires have been proven safe at speeds up to 186 miles per hour. But unless you're driving a supercar on the autobahn, it's unlikely you'll ever need tires like these.

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