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Do you know the sidewall of your car’s tire has some numbers and letters or a mixture of those? Do you know what they mean and why they are embedded there? So let's discuss why they are placed there and what they tell about your car’s tire.
The sidewall of a tire includes various important information. The numbers and letters identify dimensions and performance standards that allow you to compare tires more accurately and efficiently. The image will help you understand those numbers and letters.
Tire size in metric is presented as: Width (mm) / Aspect Ratio (%) / Construction Type / Rim Diameter (inches)
Width: The first number indicates the tire's width in millimeters.
Aspect Ratio: The second number is a percentage representing the sidewall height relative to the tire's width.
Construction Type: A letter (usually "R") designates the tire's internal construction (radial in most cases).
Rim Diameter: The last number indicates the wheel's diameter in inches.
35 is the tire height, or diameter (inches)
12.50 is the section width (inches)
R represents the carcass construction (radial)
17 is the wheel diameter (inches)
E is the load range
American standard tire sizes are typically presented as: Overall Diameter (inches) x Section Width (inches) - Construction Type - Rim Diameter (inches)
Overall Diameter: The first number indicates the tire's total height in inches.
Section Width: The second number specifies the tire's width in inches.
Construction Type: A letter (like "R" for radial or "D" for diagonal) denotes the tire's internal construction.
Rim Diameter: The final number indicates the wheel's diameter in inches.
Tire diameter is expressed differently depending on whether the tire uses American or metric sizing. For American standard tires, the overall diameter is simply the first number listed, given directly in inches. However, determining the diameter of a metric tire requires a calculation. First, convert the section width from millimeters to inches by dividing by 25.4. Then, multiply this width by the aspect ratio (expressed as a decimal) to find the sidewall height. Finally, double the sidewall height and add the wheel diameter (in inches) to obtain the total tire diameter.
Modern light truck and passenger vehicle tires predominantly utilize radial construction, denoted by the letter "R" in the tire size designation, positioned just before the wheel size. While radial tires are the most common, two other construction types exist, though less frequently encountered: bias belted tires, indicated by "B," and diagonal bias tires, indicated by "D." These letters signify the internal structure of the tire's plies, which affects its performance and durability.
Wheel size is shown in inches, regardless of whether it's a metric or American-sized tire.
The load index is a number on your tire that tells you how much weight it can safely hold. However, this number isn't the actual weight—it must be looked up on a chart. For example, load index 85 indicates a carrying capacity of 1,135 pounds.
In the early 1990s, speed ratings were introduced to indicate a tire's maximum safe operating speed. Initially, a "Z" rating signified tires tested for speeds exceeding 149 mph. While typically appearing after the load index, some manufacturers incorporated "ZR" into the tire size designation to denote a radial tire capable of these high speeds. As automotive performance advanced, the need for more precise speed ratings arose, leading to the creation of "W" for speeds of 168 mph and higher, and "Y" for speeds of 186 mph and even higher speed capabilities.
