How to Read Tire Sizes

Tire sizes may look confusing at first, but each number and letter tells you something important about the tire. A tire size shows the tire’s width, sidewall height, construction type, wheel diameter, load rating, and speed rating.

Understanding tire size markings can help you choose the right replacement tire, match an existing tire, or compare tires before ordering online. Before purchasing tires, always confirm the tire size listed on your current tire, your vehicle’s tire placard, and the product listing.

Example Tire Size: 275/45R21 110W

A common tire size may look like this:

275/45R21 110W

Here is what each part means:

• 275: The tire width in millimeters.
• 45: The aspect ratio, or sidewall height, expressed as a percentage of the tire width.
• R: Radial construction, which is the most common modern tire construction.
• 21: The wheel diameter in inches. This tire fits a 21-inch wheel.
• 110: The load index, which relates to how much weight the tire can carry.
• W: The speed rating, which relates to the tire’s maximum speed capability under proper conditions.

Tire Width

The first number in a tire size is the tire width, measured in millimeters from sidewall to sidewall. In the size 275/45R21, the tire width is 275 millimeters.

A wider tire may look more aggressive and may provide a larger contact patch, but it must be compatible with the wheel width, vehicle suspension, and overall fitment requirements.

Aspect Ratio

The second number is the aspect ratio. This number tells you the sidewall height as a percentage of the tire width. In the size 275/45R21, the aspect ratio is 45, meaning the sidewall height is 45% of the tire width.

A lower aspect ratio usually means a shorter sidewall. A higher aspect ratio usually means a taller sidewall. Changing aspect ratio can affect ride comfort, handling, speedometer accuracy, and overall tire diameter.

Construction Type

The letter after the aspect ratio identifies the tire construction. Most modern passenger vehicle tires use R, which stands for radial construction.

For example, in 275/45R21, the R means the tire is a radial tire.

Wheel Diameter

The number after the construction letter tells you the wheel diameter in inches. In the size 275/45R21, the tire fits a 21-inch wheel.

This number must match your wheel size. A tire made for a 20-inch wheel will not fit a 21-inch wheel, even if the other numbers seem similar.

Load Index

The load index tells you how much weight the tire is designed to support. In the size 275/45R21 110W, the load index is 110.

When replacing tires, the load index should meet or exceed the vehicle manufacturer’s requirements. Do not choose a tire with a lower load rating than your vehicle requires.

Speed Rating

The speed rating is the letter after the load index. In 275/45R21 110W, the speed rating is W.

Speed ratings are based on tire testing standards and indicate the tire’s speed capability under proper conditions. For replacement tires, match or exceed the speed rating recommended for your vehicle unless a qualified tire professional confirms another option is appropriate.

Common Tire Size Prefixes and Markings

Some tire sizes include additional letters before or after the main tire size. These markings provide more information about the tire’s intended use or construction.

• P: Passenger vehicle tire.
• LT: Light truck tire, often used on trucks and heavier-duty vehicles.
• XL: Extra Load, meaning the tire is designed to carry more load at higher inflation pressure than a standard-load tire of the same size.
• M+S: Mud and Snow designation.
• 3PMSF: Three-Peak Mountain Snowflake symbol, indicating the tire meets certain severe snow service performance criteria.

Where to Find Your Tire Size

You can usually find your tire size in several places:

• On the sidewall of your current tire.
• On the tire placard inside the driver-side door jamb.
• In your vehicle owner’s manual.
• On the product listing if you are replacing a tire you already found online.

The tire placard and owner’s manual are important because they show the tire size and ratings recommended by the vehicle manufacturer.

Why Tire Size Matters When Buying Replacement Tires

Tire size affects fitment, handling, ride quality, braking, load capacity, speedometer accuracy, and drivetrain performance. Choosing the wrong tire size can create safety concerns or cause rubbing, incorrect clearance, or uneven performance.

If you are replacing one tire, you should also consider the brand, tire model, tread pattern, tread depth, load rating, and speed rating so the replacement tire works well with the tires already on the vehicle.

Buying Tires from The Wheel Exchange

The Wheel Exchange offers a rotating inventory of replacement tires, new take-off tires, used tires, single tires, and tire sets. Tire listings may include details such as tire size, brand, model, quantity, load rating, speed rating, tread depth, DOT date, and condition notes.

Before ordering, review the product title, description, photos, and listed specifications to confirm the tire matches your vehicle and your existing tire setup.

Shop available tires from The Wheel Exchange.