How to Read Tire Size

What size are your vehicle’s tires? Don’t feel badly if you can’t answer that question even if you do go look.

A tire’s size is located on the sidewall — the area between the wheel rim and where the tire touches the pavement. Look for a letter like “P” or two letters like “LT” and then a string of numbers and letters, and you’ve found it. What do all of those letters and numbers mean? Continue on to learn how to read tire size.

How to Read Tire Size

The tire’s size looks like a mathematical equation, doesn’t it? “Two hundred and seventy-five divided by 35 and R to the 22nd power.” But each part of it (apart from the slash, which is silent) describes a different aspect of the tire:

1. If there’s a letter “P” starting the sequence, it’s a passenger car tire. “LT” means light truck. If there’s no letter and instead starts with a three-digit number, that’s a passenger car tire, too.
2. The three-digit number (“275” in the photo above) is how wide the cross-section of the tire is in millimeters (275 millimeters works out to about 10.8 inches). The higher that three-digit number, the wider the tire.
3. After the slash comes the first two-digit number, commonly called the series number, which indicates the sidewall’s height as a percentage of the width. So “35” in the picture above means the sidewall is only 35% as tall as the tire is wide. So, the tire is 96.25 millimeters, or about 3.8 inches, tall. This is considered a low-profile tire, which are most often seen on cars with big rims (18 inches and above). Passenger car tires go all the way up to a 75 series, and there are some truck tires as tall as 85.
4. The next letter in the sequence is almost always an “R,” and it stands for radial-ply. Non-radial tires are bias-ply tires, and they’re most commonly found on pre-1980 classic cars, motorcycles and trailers.
5. Finally, that last two-digit number tells you how big the rims are in inches. The car above has 22-inch wheels.

What About That Last Number and Letter?

The number that comes after the tire size in the inner row is load-carrying capability. It’ll be a number between 70 (rated for 739 pounds) and 126 (rated for 3,748 pounds). The “104” on the tire above means it’s rated for 1,984 pounds.

The letter is a maximum speed rating. “S” and “T” are the most common for passenger cars like family sedans with maximum speeds of 112 and 118 mph, respectively. The tire above is a “W,” which means it’s rated for a maximum of 168 mph. Some speed ratings are even higher.

Choosing the right tire is important. A tire’s size, tread and age affect your vehicle’s appearance, ride quality, handling, performance in extreme weather conditions and fuel economy. Being able to read and decode those letters and numbers on a tire’s sidewall goes a long way toward making the right choice for your needs.

Check out all the tire and wheel accessories available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on tires, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photos courtesy of Mike Hagerty.