by Biloxi Von Lutz

Created on: July 21, 2010

When the average person looks at the size on the side of the tire, it must seem like it's written in some other language. There are all sorts of numbers and letters scattered throughout the sidewall of the tire. How does a person figure out what size their tire actually is, and even more importantly, what does that size mean?

The first step is actually finding the tire size on the sidewall. This is typically listed near the rim in a format similar to this: 265/70R17 111H. Once you've located the tire size, it's time to decipher what it means.

In our example above, 265/70R17 111H, the 265 is the width of the tire in millimeters. So, this tire would be 265 millimeters wide. The middle number, 70, is called the aspect ratio, or the series. This is where things get a bit confusing. The aspect ratio is the height of the tire from the top of the rim to the top of the tread expressed as a percentage of width. Got all that?

The aspect ratio of a 26570R17 111H is 70. This means that the tire is 70% as tall as it is wide, or 185.5 millimeters. In most cases, the middle number in a tire size can just be thought of as the height to make things simple, but if you really need to know the physical measurement, you'll need to break out the calculator.

The R in the size simply denotes that the tire is a radial tire, which is a type of tire construction. Almost all passenger car tires are radial tires now, so you don't really have to concern yourself with this.

The 17 is simply the rim diameter in inches. It's odd that tire manufacturers chose to use both American and Metric systems in tire measurements, but all rims are measured in inches in the United States. The decision to use the metric system of measurement for width and height was done to make U.S. tire sizes more compatible with international standards.

The last segment of the tire size, 111H, refers to the load index and the speed rating. The 111 is essentially the load that a tire can carry at maximum air pressure. The 111 corresponds to a range on a scale, not a specific number. The H is the speed rating. This is the maximum speed that a tire can be driven for a sustained period of time. Like the Load Index it corresponds to a range of speeds. The most common speed ratings, from slowest to fastest are: S, T, H, V, W, Y. More information about the Load Index and Speed Rating can be found here.

Hopefully, this clarifies some of the mystery surrounding tire sizes. It's not exactly the most intuitive system ever devised, but it does contain an awful lot of information within a small area. In either case, it's way better than the old system of measuring tires using letters.

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