How to Understand Tire Size

You don't have to feel lost at the tire shop if you know what the numbers in the tire size mean.

Walking into a tire store can make you feel like you've wandered into a foreign country. The salespeople seem to speak an entirely different language. Not understanding tire size nomenclature could be frustrating if you don't know what each number or letter in the tire size code means. Tire makers have a uniform method of sizing and rating their products, and with a brief education, you can decipher the string of numbers and letters that helps you determine a tire's size and purpose.

Instructions

1. Read the sizing label on the side of the tire. If the string begins with a "P" the tire is rated for use on a passenger car. An "LT" designation is used for light trucks and vans. The "ST" designation means the tire should be used on boat or utility trailers, not on cars or trucks.

2. Read the first three numbers in a tire whose first letter is "P," the three numbers in front of the slash. They indicate the width of the tire in millimeters. 225 indicates a tire width of 225 millimeters, 185 indicates a tire width of 185 millimeters, and so on.

3. Read the second set of numbers, the two numbers after the slash. This is the height of the sidewall, expressed as a percentage of the tire width. On a passenger tire with a P225/55 label, the 55 indicates that the tire's sidewalls are 55 percent of the tire's width. The height of the sidewall is determined by multiplying the percentage by the width of the tire. Multiplying 225mm by 55 percent or .55 equals 140.25mm, Meaning the sidewalls are 140.25 millimeters tall.

4. Determine the tire's construction by looking for a letter following the width and aspect ratio. An "R" indicates that the tire is a radial. Most tires sold today are radial. The absence of an "R" would indicate a bias-ply tire. Due to the difference in handling characteristics, bias-ply tires should not be used on a car that also has radial tires.

5. Read the last number in the sizing sequence. This denotes the size of the wheel the tire should be mounted on. This last measurement is in inches. On a tire whose size code is P225/55-R18, the 18 indicates the tire should be mounted on an 18-inch wheel. A 16 would indicate a 16-inch wheel.

Tips & Warnings

• The two-number, single letter code following the tire size indicates the tire's service description. That is the weight and speed rating for the tire. A table for these values can be found in the resource link below.

References

• TheTireChoice.com: How to Read Your Tire Size and What it Means
• Jofat.com: Tire Size Information

Resources

• Jofat.com: Tire Service Description