How Tire Sizes Work

close up view for a tyre overall diameter label. tire with r14 radius for cars.

Putting the wrong size tires on your vehicle is like wearing a pair of shoes that don’t fit. They’re hard to put on, and they make you run weird. And just like how a shoe salesperson needs to know your shoe size before they recommend you a pair of sneakers, tire service professionals need to know what size tires will fit on your car.

You can usually find the recommended tire size in your vehicle’s owner manual. But, to the uninitiated, understanding how to read tire sizes can feel more like deciphering hieroglyphics than selecting a car part. To help make sense of how tire sizes work, we break down how to read tire sizes and what that means for your vehicle.

Reading Tire Sizes

Head out to your garage and take a look at your tires. You’ll find a series of numbers and letters that looks something like this: P215/65R15 89H. Each number and letter has a corresponding meaning regarding tire size, type, and rating. Below, we will explain the meaning of each to ensure you know what tires you’re driving on.

Tire Type

The first letter in the sequence refers to the tire type and classification. The letter P stands for passenger vehicle tire. P-class tires are used for cars, SUVs, crossovers, minivans and smaller pickup trucks. Most motorists are driving around on P-class tires.

The letters LT stand for “light truck” tires and are designed for use on vehicles that tow trailers or have a ¾ to one-ton capacity. ST stands for “special trailer”, and as you might assume, they are designed for trailers like travel or boat trailers. The final letter classification is T, which stands for “temporary”. You should only be using temporary tires until you can have your flat tire either repaired or replaced.

If you don’t see a letter at the beginning of the sequence, you most likely have a metric tire that is classified under the European system.

Tire Width

The three-digit number that follows the initial letter is for the tire’s width (side-to-side). Tire width is measured in millimetres from sidewall to sidewall. In our example, P215/65R15 89H, the tire width would be 215 millimetres.

Aspect Ratio

After the slash mark that follows the number indicating tire width is the aspect ratio, used to find out how tall your tires are. The difference here is that the aspect ratio is not a specific measurement; it’s a percentage. The percentage is found by dividing the tire’s height by its width. Let’s use our sample tire size to find out the height of our tire, P215/65R15 89H.

The number following the slash mark is 65, meaning the aspect ratio is 65. Knowing that our tire width is 215 millimetres, we can multiply 215 by .65 to get our answer. The height of our tire is 139.75 millimetres or 5.5 inches. If that number seems small, it’s because tire height is measured from the wheel rim to the top of the tread, not from the ground to the top.

Construction Type

After the aspect ratio, we find ourselves with another letter. This letter indicates the type of internal construction of your tires. There are two types of tire constructions, indicated by the letters R and B or D.

R stands for radial and is the most popular type of tire on the road today. In radial tires, their cable plies are laid radially around the axis of the tire (perpendicular to the direction of travel). These tires have superior road grip, lower rolling resistance (That means better gas mileage for you!) and durability. As you can see from our example, P215/65R15 89H, our tire is a radial type.

The letters B and D stand for “bias” and “diagonal”, respectively. These tires orient the cable plies diagonally instead of perpendicular. Bias tires are skinnier and longer than radial tires, which makes them a subpar option in terms of road grip and handling.

Wheel Diameter

Next on the list is the number that indicates the diameter of the wheel, in inches. The diameter of our wheel is going to be 15 inches because 15 is the two-digit number immediately following the R, P215/65R15 89H.

Load Index

The last two or three-digit number we need to decipher is the one that tells us the tire’s load index. The load index indicates how much weight a tire can handle. The higher the load index, the more weight it can support. However, the load index number isn’t a direct indication of the weight it can support.

For instance, let’s take a look at our sample tire, P215/65R15 89H. The load index number is 89, but that doesn’t mean 89 kilograms or 89 pounds; 89 is the code number that corresponds to the weight limit in a load index table chart. A load index of 89 means the tires can support a weight of 580 kilograms or 1,280 pounds. If you can’t find a load index chart online, ask your tire service team for guidance.

Speed Rating

The final symbol on the tire is for its speed rating. Usually, the speed rating symbol is a single letter. In our case, it’s H, P215/65R15 89H. The purpose of a speed rating is pretty straightforward, and it tells you the maximum speed it’s safe to drive with those tires. Tires with higher speed ratings are safer at higher speeds and provide motorists with more control. Just like with the load index, the speed rating corresponds to a specific number (this time its speed instead of weight) on a chart. For our tire, H means you can safely drive up to 209 km/h!

And there you have it! Now when you need to check what size of tires you are using or are deciding on which type of tire to replace your old ones with, you know what to do. At first, reading your tire’s size can feel like trying to read a language that you’ve never even heard of before. But all it takes is a little time and patience to go through the meaning of each symbol to understand what they are trying to communicate to you.

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