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Understanding the Numbers and Letters on Your Tires

Understanding the Numbers and Letters on Your Tires

When you take a look at your car’s tires, you’re going to see a series of numbers and letters that look something like 215/65 R15 or P215/65 R15. Sometimes, the slash is replaced by a dash (as in 215-65 R15).

At first glance, this series of characters might cause confusion. However, the pattern is actually easy to understand. Besides, knowing the meaning of these numbers and letters is important when it comes to maintaining and upgrading your vehicle.

So, without further ado, here’s a quick guide to deciphering the “code” on your tires:

Tire Width

The first thing you have to remember about these letters and numbers is that each represents a dimension or measurement. The first in the sequence is the tire’s width in millimetres. Taking the example above, P215/65 R15 means that the tire is 215 millimetres wide from sidewall to sidewall.

The Vehicle Type

Tires have two size designations: P-metric and Euro metric (or simply metric). P-metric tires have a letter before the tire width, which can either be P, LT, T, and ST. These letters stand for the vehicle type: passenger vehicle, light truck, temporary, and special trailer, respectively. Thus, if we again use the measurement above, a P215/65 R15 tire should only be installed on passenger-type vehicles like cars or SUVs.

LT tires can be used for vehicles that can carry heavy cargo or pull trailers, like heavy-duty pick-ups. Meanwhile, T tires are used for spares. Lastly, ST tires are only used for trailer axles and never for drive or steer wheels.

Euro metric tires, on the other hand, are tires without a letter designation (e.g., 215/65 R15). The measurement originated in Europe, which is why Euro metric tires are usually found on European cars. Do note that P-metric and Euro metric tires have the same sizes when it comes to dimensions; the only difference is a slight adjustment in load-carrying capabilities.

The Aspect Ratio

A tire’s aspect ratio is the ratio of the tire’s height to the width. Thus, a tire with a 65% aspect ratio means that its height is 65% of its width. A good thing to remember is that this number always comes after the slash or dash. Thus, if the tire is P215/65 R15, then its aspect ratio is 65%.

Most high-performance tires have a low aspect ratio, so you should choose accordingly if you prefer to make the switch. Do note, however, that you shouldn’t go too far below your car’s original tire aspect ratio so you can still maintain the original ride quality.

The Tire Construction

The letter that comes after the aspect ratio—in this case, the R after the 65—is the tire’s construction. The most common letters are R and D. R stands for “radial,” which means the internal layers are arranged perpendicular to the axis of rotation. Meanwhile, D stands for “diagonal.” A tire with a diagonal or bias ply construction means that the layers are arranged in a cross-hatch pattern.

Some tires also have “B” as their tire construction indicator, which stands for “belted.” This means that the internal layers are arranged in a criss-cross pattern (like a D tire), but with reinforcement belts under the tread. However, it’s rare to find belted tires in modern cars.

The Wheel Size

The number after the tire construction signifier is the size (diameter) of the wheel on which you can fit the tire. In our example, a P215/65 R15 will fit on a 15-inch wheel. As a reference, wheels range in diameter size from 10 all the way up to 32 inches.

There are also tires with half-inch sizes, say 15.5, 16.5, or 17.5. These are often used on heavy-duty vehicles, trailers, and box vans used to haul heavy cargo.

The Load Index and the Speed Rating

Lastly, manufacturers that also make high-speed, high-performance tires include two additional indicators: the load rating and the speed rating. They come after the wheel size and are shown as thus: P215/65 R15 91V. The 91 here is the load index, while the V is the speed rating.

In this case, a 91 load index means the tire can support up to 1,356 lbs or a little over 615 kilograms. The V speed rating, meanwhile, means the tire has a maximum speed capability of up to 240 kilometres per hour. The highest speed rating is ZR, which translates to a speed capability of more than 300 kilometres per hour.

You can consult load index and speed rating conversion charts online to get a clearer idea of what these numbers and letters mean.

 

It’s not absolutely necessary to know what all of these numbers and letters on your tires mean. However, as a responsible car owner, it’s only wise to know everything about your car. These include the tiniest details—including the indicators on your tires. When you know everything about your car, you’ll be able to take care of it better!

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