Explanation of Tire Sizes
When you look on the sidewall of your tires, you'll see something like P205/45R 17. While the string of numbers and letters might look like gibberish to the uninitiated, it really tells you everything you need to know about the size of your tires.
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Vehicle Type
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There is usually a letter or letters at the beginning of a tire size. This tells you what kind of car the tire is intended for. If it starts with a P, then that tire is intended for a passenger vehicle like a car, SUV or small pickup truck. LT means it is made for a light truck, including large pickups and vans. T designates a temporary spare, and ST mean the tire is intended for a special trailer. If there is no letter, it's referred to as "Euro-metric" and is typically the equivalent of a tire that is designated P.
Section Width
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Directly after the letter is a three-digit number--or, for a Euro-metric tire, the number will be at the beginning. This tells you the section width of the tire in millimeters. The section width is measured from the inner to the outer sidewall. If your tire is a P205/45R 17 that means the section width is 205 millimeters.
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Sidewall Aspect Ratio
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The next part of the tire size is the sidewall aspect ratio, a two-digit number directly after 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 of the section width. The higher the number, the taller the tire.
Internal Construction
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After the sidewall aspect ratio number, there is often a letter that tells you the internal construction of the tire. An R means the tire's body plies are radial and are positioned as if they are radiating out from the middle of the wheel. A D means 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 are extremely rare.
Wheel Size
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The last number identifies the rim size. If your tire is a P205/45R 17, that means it's meant for a 17-inch rim. You may also see rim sizes in expressed in millimeters, but it is pretty easy to tell which measurement is being used. It's unlikely your car would have 17-mm or 360-inch rims.
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