How To Read A Tire Sidewall

Take a quick look at any tire, and you’ll notice a series of letters and numbers around the sidewall. These might seem like random gibberish, but they tell you lots of helpful information about your tires.

So, what do all these codes mean? Here’s everything you need to know to read a tire sidewall.

Reading Metric And P-Metric Tires

Most US tires these days are P-Metric. It was introduced by a group of US manufacturers in 1977 and is now widely used throughout the country.

It’s easy to tell if your tire is P-Metric. The first character in the code will be a P!

The first character will always tell you what class of tire you’re using:

• A “P” indicates a P-Metric tire.
• An “LT” indicates a light truck tire. These tires don’t use the P-Metric system, but they’ll fit on US cars, and you read the code the same way.
• If there’s no letter, the tire is either Euro-Metric or Hard Metric. These typically aren’t used on US cars but are common on European-manufactured vehicles. They also use the same code system.

There will be three digits following the initial letter (if any). These indicate the width of the tire in millimeters. For example, if the tire says “220,” it measures 220mm from sidewall to sidewall.

After the slash, there’s a two-digit number: the tire’s aspect ratio, or the ratio of the sidewall height to the tire width. So if the two-digit number is “70”, the sidewall height is 70% of the width.

Next, there will be either an “R” or a “D.” The R is short for radial construction, which means that the plies run perpendicular to the tread. The D is short for diagonal bias construction, which means the plies run at a narrower angle to the tire tread.

The last number in the tire code is the diameter. On P-Metric and light truck tires, this diameter is documented in inches, not millimeters. If the last two digits are “20,” the tire is designed for 20-inch wheels.

Reading High-Flotation Tires

High-flotation tires are a particular case. If you’re unfamiliar with these tires, they’re wide and flat tires manufacturers typically use on off-road vehicles and performance cars. 

The first two digits in these codes display the tire diameter in inches. If the number is “36”, the tire has a total diameter of 36 inches.

Following this, you’ll see the tire width measured from sidewall to sidewall. This number can be three or four digits with a decimal point in the middle.

Next, there will be an “R” or a “D.” These have the same meaning as the corresponding letters on other tires; R for radial tires and D for diagonal bias. The last two digits give the tire diameter in inches.

Some high flotation tires are also light truck tires. In this case, the load range will be listed after the diameter, typically, the letter C, D, or E.

Other Sidewall Markings

Depending on what the manufacturer deems important, different tires will have various information markings. So far, we’ve talked about the main tire code – the big, bold letters you can read from several feet away. But what about the smaller letters and numbers all around the sidewall?

That said, there are certain things you should look out for. 

Maximum Pressure And Load Limit

In addition to the size data, you’ll also see important information about the maximum pressure and load limit. These are listed in small numbers near where the tire meets the rim.

Maximum pressure is the maximum air pressure listed in PSI. Standard load tires typically have a full inflation level of 36 PSI, while extra load tires usually have a maximum pressure of 42 PSI.

The maximum weight the tire can carry is the load limit, assuming it’s inflated to full pressure.

Load Index And Speed Rating

The load index is a little bit different from the load limit. The load limit is a single number and tells you only what the tire can carry at full pressure. On the other hand, the load index lets you know what your tire can carry at any pressure.

The load index will be a number from 0 to 150. To interpret, you’ll need to consult a load index chart. This document will often be sorted into several columns with different inflation levels and limits.

The speed rating tells you the maximum speed the tire can safely travel. This rating works by a letter system, which you can reference in a chart. Keep in mind - manufacturers test these numbers in a laboratory under ideal conditions, and they usually exceed driving speeds that would be safe on a public road.

Important Safety Markings

The DOT also requires manufacturers to include critical safety data on their tires. After the letters “DOT,” you’ll see codes telling you the manufacturer, manufacturing plant, and manufacture date.

Why is this so important? If a manufacturer issues a safety recall, you can use this information to determine whether your tires are affected.

Summary

Following this simple guide, you can easily decipher all the essential information about your tires. You’ll know how to inflate them, how much they can hold, and other valuable data. And when you need a replacement, it will be easy to order the correct size.

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