What’s my tire size?

We’ll use the following diagram as a reference to the different components of a tire size:

Service type:

There can be several different variations of the letter(s) in the beginning of the tire size. The following are the most common:

• “P” stands for Passenger. This is the most common letter you will see as it is used for tires that are meant for regular everyday passenger vehicles.
• “LT” stands for Light Truck. If you are trying to figure out whether you need a P or LT tire the key is to make sure the tire’s load carrying capacity is sufficient for the vehicle’s weight, including any cargo. The load index is discussed further below.
• “ST” stands for Standard Trailer. If you are looking for tires for your RV, 5th wheel, or a construction trailer, it is important to let the technician know you need ST tires.

Width:

The first three digits in the tire size represents how wide the tire is. This number is presented in millimeters.

Aspect ratio:

The second two digit number represents the height of the tire. This number is read is a percentage of the width. For example, if the width of a tire is 245 and the aspect ratio is 45 that means its height is 110.25mm (45% of 245).

Construction:

There are several different letters that can come after the aspect ration. This letter tells us the method by which the tire is manufactured. The following are the most common:

• “R” stands for Radial. In a radial internal construction, the cords inside the tire run along the circumference of the tire, perpendicular to the direction of travel (or axis of rotation). Radial is currently the status quo; the most common method of construction.
• “D” stands for Diagonal. In this type of tire construction, the cords run along the circumference of the tire at a 30-40° angle from bead (the part of the tire that contacts the rim of the wheel) to bead, and the layers crisscross over each other.
• “B” stands for Bias-Belted. This type of tire is similar in construction to diagonal tires except that a bias-belted tire has an additional layer above the topmost layer of ply. This additional layer is comprised of stabilizer belts, typically made of steel or other corded material applied at different angles relative to the topmost layer of ply. This offers additional support and makes the tire stiffer.

Rim diameter:

The third number in the tire size represents the diameter of your rim (or sometimes referred to as the ‘wheel’). This number is read in inches.

Load index:

The load index number in the tire size lets you know what the load capacity of the tire is. The number corresponds to specific lbs. which is summarized in the chart below.

Speed rating:

The speed rating letter in the tire size lets you know the max speed the tire is designed to endure. The letter corresponds to specific km/h or MPH, which is summarized in the chart below.

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