How To Read Tire Sizes – A 2021 Guide

Shawn
| Last Updated: March 13, 2021

If you’re in the market for a new set of tires, finding out what you need is as easy as looking at your current tires or your vehicle owner’s manual. With a set of numbers and letters, you can determine the size of your tires.

But what if you want to change your tire size? How do you know what will fit or work with your vehicle?

That’s why it’s important to understand what tire sizes mean. By discovering how to read tire sizes, you can select the right tire for your wants and needs.

So: What Do The Numbers on a Tire Mean?

Located on the sidewall, each tire has a set of letters and numbers that may seem confusing to novices. But once you know what all of these characters mean, figuring out what tire size you need for your vehicle is as simple as a quick glance at the tire. Here is a quick breakdown of the figures that give you information on the size, type, and performance level of the tire.

Tire Type

The first letter on a tire lets you know what type of vehicle the tire is manufactured for. A “P” signifies a passenger vehicle, such as a car, minivan, or SUV while an “LT” stands for light truck, which is used with trucks with 1- or ¾-ton capacity. Keep in mind that smaller trucks may use “P” rated tires, depending on their overall performance and usage.

If you don’t see a letter at the front of the tire size, this means that it’s a European tire. Replacing these tires may be more difficult, so always check your owner’s manual to ensure you get the right one.

Tire Width

After the first letter, your tire will have a three-digit number. This indicates the width of the tire in millimeters from one sidewall to the other. Besides, this is also called section width by some manufacturers.

Aspect Ratio

Following the tire width, you’ll see a slash. The two-digit number after that is the aspect ratio. This is the ratio of the tire’s height to width. For example, an aspect ratio of 70 would mean that the height of the sidewall is 70% of the tire’s width.

Construction Method

After the aspect ratio, you’ll see another single letter that indicates the construction method of the tire. In a vast majority of tires, you’ll see an “R,” which stands for radial. Radial tires feature cord plies made of fiber or polyester wounds at a 90-degree angle from the direction the tire travels. This reinforces the tire and gives it more durability.

You may also see a “D” instead of an R, which stands for diagonal. As the name suggests, this means that the cord plies are at a diagonal angle (45 degrees). Most often, these types of tires are used for trailer and motorcycle tires.

Wheel Diameter

The wheel diameter is the number following the “R” or “D,” of the construction method. This two-digit number (expressed in inches) lets you know what size wheel fits with these tires. So if you see an 18, that means it will work with wheels that are 18 inches in diameter.

Load Index

After the wheel diameter, the tire should have space. Then, you’ll see a two- or three-digit number known as the load index. The load index refers to the load that the tire can handle. For example, a load index of 97 indicates that the tire can handle a load of 1,609 pounds.

To get the total weight capacity for the vehicle, simply multiply this number by four (because of the four tires). For most passenger cars, this number ranges from 75 to 100, but heavy-duty vehicles can have a load index rating of up to 150.

Speed Rating

The very last letter on the sidewall of tires is the speed rating. Tire speed ratings range from A to Z and let you know how fast you can go with a particular tire before significant deterioration or blowout can occur. It is not an indication of how fast you should go or how well your car can handle turns or corners. Most passenger vehicles have a speed rating of either T (118 mph) or H (130 mph), although this can vary depending on the type of vehicle.

Other Tire Specifications & Markings

Besides the typical tire size indicators mentioned, you may see other numbers and markings on the tire:

DOT Tire Codes

DOT tire codes show that a particular tire is compliant with Federal Motor Vehicle Safety Standards in the United States as laid out by the Department of Transportation (DOT).

How to Read DOT Tire Codes / How to Read Tire Manufacture Date?

Although the DOT tire code consists of eight to 13 letters and numbers, the last four numbers are the ones that are the most important. These figures indicate when the tire was manufactured. The first two numbers are the week of the year (01 through 52) while the last two numbers are the year (07 equals 2007, 19 equals 2019). The first four to nine numbers and letters show where the tire was made, the manufacturer, and the manufacturer code.

You can find the DOT tire code on the inner sidewall near the wheel.

Directional Tires

Directional tires, also known as unidirectional tires, are tires that manufacturers design to only roll in one direction.

What Are Directional Tires?

Although most vehicles have non-directional tires, which provide optimal performance rolling in either direction, directional tires are designed for optimal hydroplane resistance. Because these tires have treads that channel water away, they handle better in wet conditions also.

How to Tell If You Have Directional Tires?

The easiest way to tell if you have directional tires is to look at the tread. Usually, directional tires have a solid rib in the middle of the tire and a V-shaped pattern. You can also look on the sidewall of the tire, which may have “left” or “right” listed to let installers know which side the tire goes onto the vehicle.

Turning Gibberish Into Perfect Sense: Read Tire Sizes With Ease

Although the marks on the side of the tire, including the DOT tire codes, are a bit difficult to grasp at first, you should find them easier the more you look at them. So if you need to replace a tire or all four, just do a bit of practice. You just might find that you become a tire guru after your first tire replacement or your next tire rotation.

People Also Ask

Now that you’re a bit more knowledgeable about tire sizes, you might feel a bit more confident about selecting the right one. But you still may have a few questions about certain types of tires. If this describes you, here are some of the most common questions that people also ask.

What Does LT Mean on a Tire?

Listed at the front of the tire rating, “LT” stands for light truck, which is indicative of a tire that’s rated for trucks with 1-ton or ¾-ton capacity. The ton capacity refers to the amount of weight a truck can carry including passengers, cargo, and payload. Larger passenger trucks such as a Ram 2500, Ford F-250, or GMC Sierra 2500 fall under this category.

What Does M+S Mean on Tires?

If you see M+S, MS, M/S, or M&S on a tire, it means that the tires meet the requirements and guidelines for a mud and snow tire as described by the Rubber Manufacturers Association (RMA).

What Does The T or H Mean on Tires?

T and H are two of the most common types of speed ratings on tires. These letters indicate how fast a vehicle can go on a particular tire while still safely maintaining grip in a straight line. A T is equal to 118 mph while an H is 130 mph. Remember that these ratings do not take into account turning or cornering at these speeds.

What Does ZR Mean on a Tire?

ZR is another speed rating listed on the sidewall of the tire. 

However, a ZR rating is the most versatile of any tire because it can be classified as three different speed ratings.

In short, ZR tires can double as V tires (149 mph), W tires (168 mph), or Y tires (186 mph). This means that ZR-rated tires can accommodate speeds of 149+ mph. The R stands for “radial” or the type of construction.

What Does XL Mean an Tires?

An XL on tires—usually following the speed rating—stands for extra load. These tires are typically used in vehicles that carry heavier loads, such as large trucks or limousines. Oftentimes, these tires require a higher inflation psi than standard or ordinary tires.

What Does 91H Mean on a Tire?

A 91H on a tire is a mix of the load index (91) and the speed rating (H). This means that the tire can carry a load of up to 1,356 pounds or 5,424 pounds for the entirety of the vehicle. A speed rating of H indicates that the tires can handle speeds of up to 130 mph safely.

What Does Tire Load Index Mean?

A tire load index is how much weight a tire can safely support. Load indexes range from 0 (99 pounds) to 150 (7,385 pounds). Most passenger vehicles fall between the 75 (852 pounds) to 100 range (1,764 pounds). However, heavy-duty trucks may have a load index rating above 100 pounds.

What Does Tire Aspect Ratio Mean?

A tire aspect ratio is a number on the sidewall of a tire that shows the height to width ratio of the tire. For example, an aspect ratio of 60 would mean that the height of the sidewall is 60% of the size of the width. This number is found after the slash on the tire’s sidewall.

What is Tire Speed Rating?

A tire speed rating is a letter that shows how fast a vehicle can go while safely maintaining grip or traction. The rating is typically the letter on the sidewall of a tire. This rating can range from A to Z, with T (118 mph) or H (130 mph) being the most common on passenger cars.

Shawn

An ex-salesman of industrial equipment, Shawn used to drive nearly 60K miles a year just commuting to clients. He also has a little project Miata build going on the side. Safe to say, Shawn has slain a few tires in his days. He knows all about horrid road-noise, hydroplaning risks, and how much damage a bad alignment can do to your wallet. He enjoys helping us out and Chris always values his opinion when designing something new for the website.