What Do the Codes on Your Tires Say?
If you have a car, you also have at least four tires – five if you have a real full-size spare in the trunk instead of one of those humiliating donuts. At every level, from a single car owner to a tire yard owner, there are many tires to own. The thing is, most of us do. However, very few of us understand how to replace these tires on our own, although it is not as difficult as you might think. In fact, all the information you need is literally printed on the tires you already own.
Naturally, this information is not printed in the way that it could be called “readable” – it is code. But once you understand what the code means (and master the simple math), you can figure out on your own which tire any car needs. This is especially important if you’re only replacing one tire because even small performance differences can be disastrous for your vehicle’s health. Here’s how to read your car’s tires.
Tire literacy
You may have noticed that there are a lot of inscriptions on the sidewalls of all the tires of your car – incomprehensible numbers and letters. The joke is on you: all these letters and numbers are absolutely clear. You just need to know what they mean. There are two codes you need to pay attention to. First, look at the sidewall code, and second, check the DOT (Department of Transportation) code. Let’s take two fictional tire codes and see what they mean.
Side wall code
First, look for a code that is 11-13 characters long with a slash in the middle. This is a sidewall code that describes the basic characteristics of your vehicle’s tire. Note. Tire sizes are in metric units, so for now, you’ll have to swallow your American love of the imperial system. We are going to use code P205/65R16 75H as an example.
Tire type . The first letter of the code indicates what type of vehicle the tire is for.
- P stands for passenger car. Think sedans, minivans, and most small SUVs and trucks.
- LT means “light truck” or trucks that can carry heavy loads.
- ST stands for Special Trailer and, you guessed it, is meant to be used on trailers, not cars or trucks.
- T stands for “temporary” and can be found on parts that are not full size tires.
- No letter : If your tire code does not start with a letter, you have a so-called Eurometric tire. This is a category that combines P and LT, but there are some differences in how the load index is calculated (discussed below). Generally speaking, they are considered equivalent to P-metric tires, especially if you are replacing all four. If you are not sure about something, you can still consult a professional.
Tire width . After the letter, you will find three numbers. In our sample above, it’s “205” and that’s just the width of the tire in millimeters – 205mm.
Tire aspect ratio . After the width, you will see a slash followed by two more numbers. This is the aspect ratio of your tire (what percentage is the width of the height) that is used to calculate the height of your tire using the following formula: aspect ratio/100 x width .
So in our example the tire height would be 65/100 x 205 or 133.25. This means that the height of the tire is 133.25 millimeters.
Construction type. After the aspect ratio, you will find the letter. This is a code that describes the type of tire construction.
- R means radial
- D stands for diagonal tires (for some reason).
Almost all tires you will come across will be radial tires. Bias tires are mainly used on motorcycles and trailers.
Rim size . The last two digits of the sidewall code are the rim size or wheel diameter. In our sample, this will be 16. Be careful. All of a sudden and for no good reason, we’re back to the imperial system, and it’s in inches. Why should there be a sequence? Consistency is boring.
Load index . Have you ever wondered how much weight your car tires can support? Don’t be surprised anymore! After the rim size you will find a 2-3 digit code. This is the wheel load index, which is the maximum weight your tire can support (you can look up the weight in pounds on the load index table ). Since you usually have four tires, you can calculate the maximum load on all four by multiplying this by four, of course. Since our sample has a load index of 75 that can support 852 pounds, four of these suction cups can handle about 3400 pounds.
speed rating . Tires are designed to work in certain conditions, including speed. The letter that follows the load index indicates the tire speed index. There are many speed ratings ranging from 3 mph to 186 mph or more. In our example, the “H” means the tire is rated up to 130 mph.
Point Code
There is another code on your tire to look out for: the DOT code. This code is set by the Department of Transportation and indicates that the tire meets the minimum safety requirements. It also shows who made the tire and when, which is most useful to us. The DOT code format is DOT 2620 .
Producer information will vary in length and composition. All we care about is the last four digits, which indicate when your tire was made. In our example, this was the 26th week of 2020. It is very useful to know how old your tires are.
Other information
There is a ton of other information on your bus that is easier to understand. For example, the maximum tire pressure speaks for itself. Other things are a little less important.
- Running clothes (sometimes abbreviated as TW) . This tells you how much “grip” your tires have and how quickly they wear out. This ranges from about 800, which will last forever but grab nothing, to 100, which will grab everything but wears out as you read this sentence. If you’re wondering why traction matters, you’ve obviously never hydroplaned at high speeds and seen your life flash before your eyes.
- Thrust . It measures your tire’s grip on wet surfaces, but is displayed as a letter code: AA is best, followed by A, B, and C. Most tires are rated “A”.
- temperature . This refers to how well your tires dissipate heat. Heat is really bad for your tires, so the higher the rating, the better. This will be a letter grade A, B, or C.
- M+S . If you see this, your tire is an all-season, mud and snow rated tire. If it is followed by the letter “E”, you have a winter tire.
- Arrows . See the arrows on the tire? Congratulations, you have a unidirectional tire. It simply means that the tire is designed to fit in one direction. Most tires are unidirectional, but if you can’t see the arrows, don’t worry.
That’s all there is to know about tire codes. Now that you’re tire savvy, you can change your own tires or engage your mechanic in a conversation about the tires you know mechanics love.