How Are Car Tires Graded?
Tire ratings are the most important factor to consider when purchasing replacement rubber. These ratings will tell you the size, speed rating and intended performance range of your tire.
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Sizing
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The last number on a tire's information strip is the rim size. A P275/60/ZR15 tire fits a 15-inch rim.
Width
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The first three digit number in the rating strip is the tire's width in millimeters. One inch equals 25.4 millimeters, so a 185 tire is about 7.5 inches wide.
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Aspect Ratio
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The two digit number in the center of the information strip is the tire's aspect ratio (profile), which is its side-wall height divided by width. Tires with lower (below 50) profiles have shorter sidewalls and are geared toward handling.
Speed Rating
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The two letters before the rim size indicate tire speed rating, which starts at M-rated (81 miles per hour) and increases through Z-rated (149+ mph). W-rated is 168 mph, and Y-rated is the highest at 186-plus mph.
Tire Types
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An information strip beginning with a P indicates a passenger car tire. LT stands for light truck (SUV), and ST stands for special trailer. Trucks and SUVs should only use heavy-duty LT-rated tires, and ST tires should never be used on anything but trailers.
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References
- Photo Credit Image by Flickr.com, courtesy of Saquan Stimpson
