How do you know the important stuff about a tire? Like, what’s inside it and what size it is? It's easy. This information is written on the tire sidewall.
The image above shows two tire sizes you might see. Let's break down the numbers.
The first, a 130/90, is a common size. The 130 represents the width across the face of the tread in millimeters. This may not be exactly identical from one brand of tire to another. Each manufacturer varies slightly, and the curvature of the tire’s profile can affect the overall measurement, but the tolerances are close enough that one will fit where another goes, as long as you're sticking to stock sizes.
The second tire example, in the alphanumeric system, conveys the width with the letter “T.” Tire width charts are widely available in tire catalogs and online, so to save space, we’ll omit them here. (For those curious, the “M” indicates “Motorcycle.” Every alphanumeric tire you’ll run into will start with an “M.”)
The 90 represents the aspect ratio. The aspect ratio is the height of the sidewall expressed as a percentage of the width. Thus, this tire has a side wall height of 90 percent times 130 mm or 117 mm.
The next item you’ll see is the rim size expressed in inches. Were this tire a radial, we would see a capital letter “R” separating the aspect ratio and rim size. Since that is not the case here, you can be certain this is a bias-ply tire. Were this tire a bias-belted tire (like a bias-ply tire with additional, stiffening layers of fabric placed over the body plies), a capital letter “B” would separate the aspect ratio and rim size. It's easy to see this tire is made to be mounted on a 16-inch wheel.
Other information shown are speed ratings and load ranges. Load ranges give the maximum weight a tire can carry, and speed ratings list the maximum speed at which the maximum load can be carried. Charts to find a given load or speed for a particular letter or number again are available in a multitude of places, so we’re not including them here.
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