How to Read Tire Size Conversions
Reading tire size conversions for the first time may seem a bit confusing. They are really simple codes, stamped onto the sidewall of any tire, used by manufacturers and mechanics to describe the size, purpose and the limitations of each tire. Nearly all tire conversion numbers have been standardized since 1991. With some insight into what the letters and numbers mean, you will have a much easier time choosing new tires that are best suited for your vehicle and your driving habits. Use the following steps to read and decipher the tire conversion numbers for your vehicle.
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Instructions
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Look for the tire conversion numbers on the sidewall of your tire. As an example, the conversion series would resemble something like this: "P220/55R16 71S."
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Look for the Service Type for the tire. This is one or two letters usually found at the beginning of the tire size. In our example, "P" stands for Passenger. The letters "LT" stand for Light Truck. The letters, "ST" stand for Special Trailer, which means the tire was designed for utility trailers, car trailers or boat trailers. Sometimes the Service Type letters will be at the end of the tire size. In that case, "LT" would mean the tire is designed for vehicles carrying heavy loads or trailers, or for off-road usage. The letter "C" at the end would stand for Commercial, meaning the tire is to be used for commercial vans or trucks.
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Look for the Section Width. This is the first three numbers following the Service Type letter. They tell you the width of the tire from the widest points of the inner and outer sidewalls when the tire is fitted onto the vehicle and pressurized. This number is usually in millimeters. In our example, "220" means a tire width of 220 millimeters.
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Convert the section width from millimeters to inches by dividing by 25.4. For example: 220 millimeters divided by 25.4 equals 8.66 inches.
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Look at the Sidewall Aspect Ratio. This is the two-digit number after the Section Width. The number represents the height of the sidewall from the tread to the rim, as a percentage of the Section Width. A higher number means a taller sidewall. In our example it is 55 percent.
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Multiply the Section Width by the Sidewall Aspect Ratio. Again, using the example above, 220 millimeters multiplied by 0.55 equals 121 millimeters, or 4.76 inches.
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Look for the Internal Construction code. This is the next letter after the Sidewall Aspect Ratio. In our example, it is the letter "R", which means it is a Radial tire. The letter "D" means it is a Diagonal bias ply construction, and "B" means it is a Belted tire.
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Look for the Rim Diameter. The next number shows the diameter of the rim the tire was designed for. This is usually expressed in inches. In our example it is "16." However some older models may be expressed in millimeters. If it is in millimeters, it will be a three digit number, while inches are in two digits. Some light trucks, trailers and vans use rim diameters in half inches, in which case you would see a decimal point in the number.
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Look for the Load Index. In our example it is "71." The higher the number, the heavier the load the tire is able to handle. Most passenger cars have a load index between 70 and 110. A load index of 71 is designed for loads up to 761 pounds. A load index of 110 is designed to carry loads up to 2337 pounds.
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Look for the Speed Rating of your tire, following the Load Index. This is designated by a letter. In our example it is the letter "S," which is for family cars has a speed rating of 112 mph. The letter "V" has a higher speed rating of 149 mph and is designed for sports cars. There are over a dozen different speed rating codes.
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Write down the information you have gathered here and use this information when shopping for new tires for your vehicle.
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