What do all the Letters and Numbers Mean on the Sidewall of Your Tires?

ByMohtashim Khan March 19, 2025

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Aside from the brand (Goodyear, Michelin, etc.) of their tires, most people are unfamiliar with the important information on the tire's sidewall and how to understand what it all means. Understanding tire numbers can help you choose the right tires for your vehicle and ensure your safety. So, what do the numbers mean? Let’s break them down below!

Tire Sizes Explained

1. The first letter of each sequence tells what the tire class is.

• P stands for passenger vehicle tire. These include cars, SUVs, crossovers, minivans, EVs, and smaller pickup trucks.

• LT stands for light truck. It is used for trucks and SUVs carrying heavy loads, towing trailers, or vehicles that often drive on rough or rugged terrain.

• ST means special trailer tires. These tires are designed for trailers, campers, boat trailers, and utility trailers.

2. After the tire class, you will see three numbers representing the tire width in millimeters, measured sidewall to sidewall.

3. The aspect ratio is two digits after the slash and is a percentage that tells you the ratio of the tire’s height to width.

4. Next, you will find a letter describing the tire’s construction type.

• R is the most commonly used and stands for radial. This means that the piles, or layers, run radially across the tire or perpendicular to the direction of travel.

• D stands for diagonal, meaning the piles are diagonal or crisscrossed pattern.

5. The wheel diameter comes next and is the diameter of the wheel (rim) the tire fits, measured in inches. For example, if the number is 15, the tire would fit a rim with a 15-inch diameter. To help you figure this out, you could use a tire diameter calculator.

Other Tire Numbers Explained

Load Index & Speed Ratings

Following the tire size, there will be a gap and then a set of numbers followed by a letter. The two- or three-digit number will indicate the tire load index, which indicates how much weight the tire can safely support when inflated. The load index can range from 0 to 150, but most passenger vehicle tires range from 75 to 100. The higher the load index, the more weight the tire can carry. The letter that follows is the speed rating. This indicates the maximum speed that a tire can safely reach and maintain. For example, an “H” means the car can travel at a maximum speed of 130 mph.

US DOT & Safety Standard Markings

The DOT (Department of Transportation) and safety markings provide information about how, when, and where your tires were made and if they comply with federal regulations. The DOT certifies that the tire meets or exceeds the USDOT’s safety requirements. If a tire doesn’t have this designation, it is not legal for street use. After the DOT designation, two letters will indicate the plant code. These characters identify the tire’s manufacturer and the site where it was manufactured. The last four digits indicate the week and year of manufacture.

Treadwear, Traction, and Temperature Markings

The treadwear grade demonstrates how long the tire tread is expected to last compared to a standard, or control, tire with a 100 rating. Higher numbers indicate longer tread life, so a rating of 400 would mean that the tire should last four times longer than the control tire.

Traction grade comes next and indicates the tire’s ability to stop on a straight, wet surface. Grades include AA, A, B, and C, with AA being the highest and C being the lowest. A higher grade means that the tire performed better in wet conditions.

The temperature grade indicates the tire’s ability to dissipate heat and resist temperature buildup under high-speed conditions. These ratings range from A (highest) to C (lowest).

Now that you understand the various letters, numbers, and markings on your tire sidewall, you can use them to make informed decisions about your vehicle's safety, performance, and maintenance. When it’s time for new tires, you’ll have the knowledge to choose the perfect ones for your vehicle's needs.