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How to Read Your Tire Size & What It Means

Ever wonder how to read your tire size, and what those numbers and letters printed on your tires’ sidewalls actually stand for? Tires are one of the most important parts of your vehicle, so you should know as much as you can about them, and there is no better place to start than understanding how to read tire size.

Finding the tire size

This part is pretty simple. You can find the tire size listed right on the sidewall of the tire. It will be a combination of numbers and letters and will look something like:

P205/55R16 91H

(We'll use this size as the example for the rest of this article.)

Let's start by breaking down the example tire size piece by piece.

Service Type - P

Most tire sizes are preceded by a letter that identifies the type of vehicle or type of service that the tire is designed for. The letter is typically a "P" signifying "P-metric," which are tires intended for passenger vehicles. Another common service type is "LT" which signifies Light Truck, and are designed for vehicles that can carry or tow heavy cargo such as pickup trucks, sport utility vehicles, and full-size vans. Keep in mind that not all tire sizes begin with a letter. Tire sizes that do not begin with a letter are referred to as "Metric" or "Euro-metric" sizes. Metric sizes are primarily used on European cars, but they are also used on vans and sport utility vehicles.

Section Width - 205

Following the Service Type, we have a three digit number called the Section Width. This number is basically the tire width measured in millimeters. In our example we have a section width of 205, which means that the width of the tire is 205 millimeters (from inner sidewall to the outer sidewall).

Aspect Ratio - 55

After the section width there is typically a slash and a two digit number called the Aspect Ratio. The aspect ratio is a number that indicates the height of the sidewall (from the edge of the inner rim to the tread) in relation to the section width. In our example we have an aspect ratio of 55, this means that the height of the sidewall is 55% of our section width of 205mm. The higher the aspect ratio - the higher the sidewall, the lower the aspect ratio - the lower the sidewall. You may have heard the term "low-profile tires;" these are tires with a low aspect ratio, meaning they have very low or "thin" sidewalls.

Internal Construction - R

The letter after the aspect ratio signifies the internal construction of the tire. In our example (as with almost all tires sold today) there is an "R" which stands for "Radial".

Wheel Diameter - 16

Next, we have the two digit number after the "R". This number is the wheel diameter measured in inches, and refers to the size rim that the tire fits on. In our example the tire would fit on a 16" rim.

Service Description - 91H

Although not really part of the tire size, there is a number and letter group called the Service Description that can be found following the wheel diameter. The service description is comprised of a two or three digit number and a letter. The number is the Load Index, which signifies the load carrying capabilities of the tire. In our example we have a load index of 91, which means that the load capacity of the tire is equal to 1,356 pounds. The letter after the load index is the Speed Rating and refers to the maximum speed that a tire has been tested to withstand. The "H" speed rating in our example means the tire is rated for a maximum speed of 130 MPH.

 

Tips and Facts

Tip: When replacing your tires, don’t mix different types, sizes or tread patterns.
Tip: If you ever buy two new tires instead of four, replace the rear tires. This will give you better control in wet conditions.
Fact: B (belt) construction tires are rare antiques that are no longer produced.
Fact: The number on the sidewall is the max pressure the tire can work with – not the recommended pressure for the vehicle.
 

 



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