How To Read Tire Size

Learn how to read tire size markings printed on the sidewall of your vehicle's tires. This guide explains the meaning of each number and letter, how to interpret them, and how to use that information for safe tire maintenance and replacement.

Fast Answer

Tire size is a code printed on the sidewall, like P215/65R15, that indicates width, aspect ratio, construction, and wheel diameter.
To read it: the first number is width in millimeters, the second is aspect ratio (sidewall height as a percentage of width), the letter indicates construction, and the last number is wheel diameter in inches.

Set-up ready What to have on hand

Step-by-step Guide format

Device-specific Check official settings

Before You Start

Park your vehicle on a level surface and turn off the engine. Engage the parking brake to ensure safety.
Find a well-lit area where you can clearly see the sidewall of your tires. If needed, use a flashlight.
Locate the tire size markings on the tire's sidewall; they are usually printed in a series of letters and numbers, often starting with a letter like P or LT.
Have a pen and paper or a smartphone handy to write down the tire size and other important numbers for reference.

Check first: Always make sure the vehicle is on a stable, level surface and the parking brake is engaged before inspecting tires. Never stand too close to moving tires or attempt to read the size while the vehicle is moving.

Step-by-Step Instructions

Locate the Tire Size Markings on the Sidewall

Look at the sidewall of your tire, the outer part that touches the air. The tire size is usually printed in raised letters or numbers, often on the side facing outward. It might be near the rim or near the tread edge. If you can't see it, rotate the tire slightly by moving the vehicle forward a bit, but only on level ground with the parking brake on. The markings consist of a combination of letters and numbers, such as P235/75R15. The sequence might also be preceded by a brand name or other codes. Use a flashlight to clearly read it. Write down the entire sequence exactly as it appears, including any letters and numbers. If you have difficulty, you can also check your vehicle owner's manual or the sticker on the driver's side door jamb, but the sidewall is the most accurate for your current tires.

Tip: If the sidewall is dirty, wipe it with a rag to make the markings easier to read. Avoid using harsh chemicals that might damage the rubber.

Identify the First Letter for Tire Type

The first letter in the sequence indicates the type of tire. Common ones: P for passenger vehicle, LT for light truck, T for temporary spare, and ST for special trailer. Sometimes there is no letter, meaning it's a metric tire commonly used in Europe. This letter tells you if the tire is designed for regular cars, trucks, or as a spare. For example, if you have an SUV, it might have LT tires. This information is important because it helps you choose the right replacement tire for your vehicle's intended use. If you're not sure what type your vehicle needs, consult your owner's manual or a tire professional. Make a note of this letter, as it will help you when purchasing new tires.

Tip: Tires without a leading letter are often metric tires, but they are less common in the US. Always match the type letter when replacing tires.

Decode the Three-Digit Width Number

The next three-digit number, such as 235, represents the tire's width in millimeters from sidewall to sidewall, not including any raised letters or protective ridges. This measurement is taken at the widest point of the tire when mounted on a specified rim. For instance, 235 means the tire is 235 millimeters wide. This width determines how much surface area the tire has on the road, affecting traction and handling. To check that you have the correct width, compare it with the recommended size in your owner's manual. A wider tire may provide more grip but can also affect fuel economy and speedometer accuracy. If you're considering a different width, always consult a professional, as it can impact safety.

Tip: The width number is always in millimeters, even in the US. So, a tire marked 235 is 235 mm wide, regardless of other units.

Understand the Aspect Ratio Number

After the slash, you'll see a two or three-digit number, such as 75. This is the aspect ratio, which is the height of the sidewall as a percentage of the tire's width. For example, 75 means the sidewall height is 75% of the tire's width. So, for a 235/75 tire, the sidewall height would be 176.25 millimeters. This number affects ride comfort and handling: a lower ratio means a shorter sidewall, which gives better cornering but a rougher ride. A higher ratio gives a taller sidewall with more cushioning but more body roll. Compare this number to the recommended size to ensure proper fitment for your vehicle, as it can affect speedometer accuracy and suspension clearance.

Tip: Aspect ratio is a percentage, so always treat it as such. For example, 70 means 70% of the width, not 70% of the tire diameter.

Recognize the Construction Letter and Rim Diameter

The letter R stands for radial construction, which is the standard for most modern tires. Other letters include D for diagonal bias ply and B for belted bias. After the letter, you'll see a number like 15, which is the wheel diameter in inches. This tells you what size rim the tire fits. For example, R15 means it's a radial tire for a 15-inch wheel. This number must match your wheel size for a safe fit. To verify, check the stamping on the wheel itself or in your vehicle manual. Using the wrong rim diameter can lead to tire failure and unsafe driving conditions. Note that the diameter is always in inches, even if other measurements are metric.

Tip: Radial tires have a belt of fabric embedded in the rubber for better stability. 'R' is the most common construction type.

Review the Rest of the Sidewall Markings

After the basic size, you may see additional markings like load index and speed rating, e.g., 95H or 102T. The load index is a number from 71 to 110 that indicates how much weight each tire can carry safely. The speed rating is a letter from L to Y that shows the maximum safe speed for the tire. These are critical for safety. To check if your tire meets your vehicle's requirements, look for a placard in the driver's door jamb or the owner's manual for the recommended load index and speed rating. If your current tires have a lower rating than specified, they may be unsafe. Also, note the DOT code near the bead, which includes the manufacturing date and plant. You can use this to check tire age.

Tip: The load index and speed rating are not optional. Use the ratings printed on the tire, but always match or exceed the vehicle's original specifications.

Quick Reference

Situation	Action	Why it helps
You are at a tire shop and need to replace a tire but don't have a manual.	Write down the full sidewall code from the old tire, including all letters and numbers, and match it with the new tire to ensure the same size and type.	The sidewall code provides the exact specifications recommended by the vehicle manufacturer for safe operation.
You are checking if your tires are underinflated but want to know the correct pressure.	Locate the tire pressure placard on the driver's side door jamb or inside the fuel door, as it lists the recommended PSI for your tire size.	Pressure recommendations vary by tire size and vehicle load; using the correct PSI ensures optimal contact, wear, and safety.
You are purchasing new wheels and need to ensure the tire fits the rim.	Compare the rim diameter number (after the letter R) with the wheel diameter stamped on the rim or listed in the vehicle manual to confirm they match.	Mounting a tire with the wrong rim diameter can cause bead failure and rapid tire wear, leading to hazardous driving conditions.

Common Issues

Read the tire size incorrectly due to other numbers printed on the sidewall.: Focus on the sequence that includes the slashes and letters like P215/65R15. Ignore load index and speed rating numbers unless you are checking those specific characteristics.
Mixing tire sizes on the same axle can affect handling and safety.: Ensure that all four tires have the same size and type. If you must replace only one tire, it is best to match the size and type of the other tire on the same axle.
The tire size markings are faded and hard to read after years of use.: Use a tire crayon or chalk to outline the raised letters and numbers. If still unreadable, refer to the vehicle owner's manual or the info placard on the driver's side door jamb.

Advanced Tips

Check your owner's manual for the exact tire size recommended for your vehicle, as it may differ from what's currently installed. Stick to the manufacturer's specifications for optimal safety and performance.
If you are changing tire sizes, consult a professional tire dealer to ensure the new size fits your vehicle's suspension, brakes, and speedometer calibration.
Use the DOT code on the tire sidewall to determine its age. Tires older than six years should be inspected by a professional, regardless of tread depth.

Final Checklist

Park on a level surface, engage parking brake, and read the full size code from the sidewall.
Identify the type letter, width, aspect ratio, construction, and rim diameter from the code.
Compare your findings with the recommended specifications in the vehicle manual or door placard.
Record the load index and speed rating to ensure your tire matches your vehicle's requirements.

FAQ

What does the first letter like P or LT mean in a tire size?

The first letter indicates the tire's intended use: P for passenger vehicles (cars, minivans, light SUVs), LT for light trucks (pickups, larger SUVs), T for temporary spare, and ST for trailer tires. This letter helps you choose the right tire for your vehicle's load and speed capabilities.

Can I use a tire with a different aspect ratio than my original?

Yes, but it changes the sidewall height and diameter, which can affect speedometer accuracy, ride comfort, and handling. Always ensure the new tire's overall diameter is within the manufacturer's acceptable range, and consult your owner's manual or a professional to avoid clearance issues.

Where can I find the correct tire size if the sidewall is damaged?

Check the tire information placard on the driver's side door jamb, glove box, or inside the fuel door. You can also refer to your vehicle's owner's manual or contact a tire professional who can identify the size based on your vehicle's make and model.