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The sidewall of every tire is printed with a code that looks intimidating but actually tells you everything you need to know about size, capacity, and speed rating. Here’s how to read it.
1. The size code: width, aspect ratio, and wheel diameter
A code like 225/65R17 breaks down into three parts: 225 is the tire’s width in millimeters, 65 is the aspect ratio (the sidewall’s height as a percentage of that width), and 17 is the wheel diameter in inches the tire is built for. The “R” simply indicates radial construction, which is standard on virtually all passenger tires today.
2. The load index tells you how much weight the tire can carry
A two- or three-digit number after the size, like the 91 in P225/65R17 91H, is the load index — a code that corresponds to a specific maximum weight the tire can safely carry per tire, not for all four combined. Load index charts are widely available and let you convert that number into an actual weight figure to compare against your vehicle’s requirements.
3. The speed rating is a letter, not a suggestion to test
The letter right after the load index (the H in the example above) is the speed rating, indicating the maximum speed the tire is rated to sustain under proper conditions. It’s not a target speed to aim for — it’s a ceiling that matters for how the tire performs at highway speeds and in hot weather, and mismatching it against your vehicle’s manufacturer recommendation can affect handling.
4. The DOT code tells you the tire’s actual age
A separate code starting with “DOT” ends in four digits — the first two are the week of manufacture, the last two are the year. A tire reading 2223, for instance, was made in the 22nd week of 2023. Tires degrade with age even with light use, and many manufacturers recommend replacement around 6 to 10 years regardless of tread depth, so this code matters even on a tire that looks fine.
5. Match replacements to your original tire’s full code, not just the size
Two tires can share the same size number but have different load indexes or speed ratings, and using a lower-rated replacement than your vehicle came with can affect both handling and, in some cases, insurance or warranty terms. When replacing tires, matching the complete code — not just the size everyone quotes casually — keeps the vehicle within its designed specifications.
The short version
The size code breaks into width, aspect ratio, and wheel diameter. The load index converts to a maximum weight per tire using a published chart. The speed rating is a ceiling, not a target. Check the DOT code for the tire’s actual manufacture date, since age matters even with light use. And match your full tire code, not just the size, when you replace them.