How to Check Tire Size

Making sure to install the right tires on your vehicle is important to the overall performance and safety. Your choice in tires should reflect your driving conditions and your preferences of longevity, response, and handling. You should understand what tire size is right for your vehicle, to be able to make the right choice when it is time to buy tires.

All the information you need is available either in the vehicle’s owner’s manual, or listed on the vehicle placard located on the side of the driver-side doorframe. If you still own the original equipment tires for your vehicle, you can also check the sidewall of the tire for its size. No matter where you locate your tire size, you will have to decipher a sequence of numbers and letters to understand the factors that go into selecting the right tires for your needs.

While actual, physical measurements like width and diameter are factors in finding the right tire size, there are additional details you should consider. Things like load index and speed rating are also important to finding the correct tires.

How to Read Tire Size

One of the most common tire sizes is P205/55R16 91H. We will use this size as an example to help explain the meaning of each character in the sidewall code.

Tire Type
Many tire sizes begin with a letter that denotes the tire type. In the case of our example, “P” stands for “P-metric,” meaning the tire is a passenger tire. P-type tires are designed for cars, crossovers, and light duty pickups.

“LT” tires are designed for light trucks, and can support ¾ and 1-ton pickup trucks, SUVs, and full-size vans. LT tires boast heavy duty construction that make them more resistant to extreme conditions and vehicle loads.

“T” is the designation for a temporary spare tire. These tires should only be used in an emergency situation, or to get you to a tire shop.

Some tire size codes do not begin with a letter. This omission usually means the tire is a “metric” tire. Originating in Europe, metric tires have the same dimensions as P-metric tires, but have been developed using world recognized European standards.

Tire Width
Immediately after the letter, you will see three digits representing the tire width in millimeters. This measurement refers to the distance between the furthest points on the tire’s two sidewalls. A larger width denotes a wider tire. Our example tire has a width of 205 mm.
Aspect Ratio
After the width, you will find a slash and a two-digit number. That number refers to the aspect ratio, which provides an idea of the tire height. The aspect ratio expresses the tire sidewall height as a percentage of the width. A high aspect ratio means the tire has a larger sidewall, while a lower aspect ratio denotes a lower sidewall. Our example tire has an aspect ratio of 55, meaning that the sidewall height is 55% of the width.
Tire Construction
The letter following the aspect ratio indicates the tire construction type. This letter may be either “R” or “D.” R denotes a radial tire, which is composed of plies that run perpendicular to the bead and ground. D denotes a bias ply tire, which is made from plies that criss-cross in layers. Radial tires are much more commonly used on passenger vehicles today.
Rim Diameter
A number after the letter indicates the rim diameter. It refers to the size of the wheel the tire fits on. Rim diameters often reflect common wheel sizes, including 14, 15, 16, and 17-inch wheels. Our example tire has a rim diameter of 16, meaning that it can only be mounted on a 16-inch wheel.
Load Index
After the rim diameter, you will find a number representing the vehicle’s load index. The load index indicates maximum load carrying capacity. This value has a corresponding load, in pounds, which can be found on the standardized load index table. Our example tire has a load index of 91. This means it can support 1356 pounds at maximum inflation, as per the load index table.
Speed Rating
Tire size codes end with a single letter that represents the speed rating. This letter denotes the tire’s maximum speed capability, as determined by laboratory testing. Our example tire holds an “H” speed rating, which means it has been tested to safely reach and sustain speeds up to 130 miles per hour (mph).

Learn more about Tire Speed Ratings.

Come to Discount Tire if you would like assistance checking tire size and reading sidewall codes. Our expert technicians can answer questions about tire dimensions and help you choose the right tire for you.