Sizing Automotive Tires
When sizing automotive tires, the string of numbers and letters on the sidewalls of automobile tires contains all the information you need to find replacement tires. The alpha-numeric code on the tire of the typical family sedan begins with "P," which stands for "passenger" car.
The following number (such as 215) indicates the width of the tire (in millimeters). Then, there is a slash followed by the tire's "aspect ratio," which is the ratio of the tire's height to width (expressed as a percentage).
The lower the aspect ratio is, the lower the profile of the tire is and the quicker its steering response.
Since most tires are radials, an "R" then follows, after which there is a number indicating wheel diameter (in inches).
TIP: The final numbers and letter in a tire's size description (for instance, the "91S" in P225/50R16 91S) represents the tire's "service description," which identifies the tire's Load Index and Speed Rating on all but Z-speed-rated tires.
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