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If you own a car, you also own at least four tires—five if you’ve got an actual full-size spare in the trunk instead of one of those humiliating donuts. At every level from single-car owner to tire yard proprietor, there are many tires to be owned. The point is, most of us do. Still, very few of us understand how to replace those tires on our own, even though it isn’t as complicated as you might think. In fact, all the information you need is literally printed on the tires you already own.
Naturally, that information isn’t printed in what might be called a “human readable” manner—it’s a code. But once you understand what the code means (and master a little simple math), you’ll be able to figure out what kind of tire any vehicle needs all on your own. This is especially crucial if you’re only replacing one tire, because even tiny differences in specs can have pretty disastrous results on your car’s health. Here’s how to read your car’s tires.
Tire literacy
You may have noticed that the tires on your car all have a lot of writing on their sidewall—inscrutable numbers and letters. Joke’s on you: all these letters and numbers are totally scrutable. You just need to know what they mean. There are two codes you’ll need to pay attention to. First, look at the he sidewall code and second, check out the DOT (Department of Transportation) code. Let’s take two made-up tire codes and walk through what they mean.
Sidewall code
First, look for a code that’s 11-13 characters long, with a forward slash in the middle. That’s the sidewall code that describes the fundamentals of the tire on your car. Note: Tire sizes are in metric, so you’re going to have to swallow your American love of the imperial system for the time being. We’re going to use the code P205/65R16 75H as an example.
Tire type. The first letter of the code describes what type of vehicle the tire is meant for.
- P indicates a passenger vehicle. Think sedans, minivans, and most small SUVs and trucks.
- LT stands for “light truck,” or trucks that can carry heavy loads.
- ST stands for “special trailer” and, you guessed it, is intended for use on trailers, not passenger vehicles or trucks.
- T stands for “temporary” and will be found on spares that aren’t full-size tires.
- No letter: If your tire’s code doesn’t start with a letter, you’ve got what’s called a Euro-Metric tire. This is a category that straddles P and LT, but there are some differences in how the load index (discussed below) is calculated. Generally speaking, they’re considered equivalent to P-metric tires, especially if you’re replacing all four. If you’re not sure about something, you can still check with a professional.
Tire width. After the letter, you’ll find three numbers. In our sample above, this is “205” and it’s just the width of the tire in millimeters—205mm.
Tire aspect ratio. After the width you’ll see a forward slash, followed by two more numbers. This is your tire’s aspect ratio (what percentage the width is of the height), which is used to calculate your tire’s height using the following formula: aspect ratio/100 x width.
So using our example, the height of the tire would be 65/100 x 205, or 133.25. That means the tire is 133.25 millimeters high.
Construction Type. After the aspect ratio you’ll find a letter. This is a code describing the tire’s construction type.
- R stands for radial
- D stands for bias tires (for some reason).
Almost all the tires you’ll encounter will be radial tires. Bias tires are mainly used on motorcycles and trailers.
Rim size. The last two numbers of the sidewall code are the rim size or diameter of the wheel. In our sample, that would be 16. Be careful here. Suddenly and for no sane reason, we’re back in the imperial system and it’s in inches. Because why should there be consistency? Consistency is boring.
Load index. Ever wondered how much weight your vehicle’s tires can handle? Wonder no more! After the rim size, you’ll find a 2-3 digit code. This is the wheel’s load index, which is the maximum weight your tire can handle (you can look up the weight—in pounds—on a load index chart). Since you’ve usually got four tires, you can figure out the max load of all four by multiplying this by, of course, four. Since our sample has a load index of 75 which can handle 852 pounds, four of those suckers can handle about 3,400 pounds.
Speed rating. Tires are engineered to operate under specific conditions, and that includes speed. The letter that follows the load index is the tire’s speed rating. There are a lot of speed ratings ranging from a dawdling 3 mph to 186 mph or more. In our sample, the “H” indicates the tire is rated for speeds up to 130 mph.
DOT Code
There’s another code on your tire worth paying attention to: The DOT code. This code is mandated by the Department of Transportation and indicates the tire has met minimum safety requirements. It also shows who manufactured the tire and when, which is most useful for us. The format of the DOT code is DOT 2620.
The manufacturer info will vary in length and composition. All we care about is the last four digits, which indicate when your tire was made. In our sample’s case, it was the 26th week of 2020. Knowing how old your tires are is very useful information.
Other Info
There’s a ton of other info on your tire that is either easier to understand. The maximum tire pressure is pretty self-explanatory, for example. Other things are slightly less crucial.
- Treadwear (sometimes abbreviated TW). This tells you how much “grip” your tires have and how fast your tires will wear out. This ranges from about 800, which will last forever but grip nothing, to 100, which will grip everything but wear out while you’re reading this sentence. If you’re wondering why grip matters, you’ve obviously never hydroplaned at high speeds and seen your life flash before your eyes.
- Traction. This measures your tire’s traction on a wet surface, but is rendered as a letter code: AA is best, followed by A, B, and C. Most tires will be rated “A.”
- Temperature. This refers to how well your tires dissipate heat. Heat is really bad for your tires, so a higher rating is better. This will be a letter grade of A, B, or C.
- M+S. If you see this, your tire is an all-weather tire rated for mud and snow. If it’s followed by an “E,” you’ve got a snow tire.
- Arrows. See some arrows on your tire? Congratulations, you’ve got an unidirectional tire. That just means the tire is engineered to be installed in one direction. Most tires are unidirectional, but if you don’t see arrows don’t freak out.
That’s all there is to know about tire codes. Now that you’re tire literate you can replace your own tires—or engage your mechanic in conversation about tires, which you know mechanics love.
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