Q: How can I determine my tire's size?

Most of us never think about what size our tires are; it’s something that usually remains out of mind until getting ready to purchase tires, or thinking about changing tire size. The size of your tires can be easy to find if you know what you are looking for, and what you are reading. Knowing what the numbers on the sidewall of the tire mean can make you a more informed tire buyer, and keep you from getting ripped off.

You can either get the tire size from the placard sticker on the inside of the driver front door jamb, or simply read it off of the sidewall of the tire itself. The most common tire out there today is known as a P metric. The P stands for "passenger tire". The numbers after the P are what indicate the tire size. The first number is in relation to the tire’s width. This number indicates how wide the tire is from sidewall to sidewall in millimeters. The second number shown after the width is in relation to the height, but it’s a little bit different. This number is known as the "aspect ratio", which is a percentage of the height. To illustrate, if you have a tire that is 255 millimeters wide, and has an aspect ratio of fifty, then the height of the sidewall is 50% of the width of the tire, or 127.5 millimeters.

The last number you will see on the tire size marking is rim size. This number tells you what size wheel the tire will fit on. A tire with an 18 after the aspect ratio means that that tire will fit on an 18” wheel, and only an 18” wheel; you cannot fit a tire onto the wrong size wheel as it can cause serious problems. It’s a good idea to use the same size tires that the vehicle came with, for a variety of reasons. Also, keep in mind that there are other tire types out there, besides P metric, that have a little different information, although it is not too common.