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What Do the Numbers in Your Tire Mean?

Your tires have a lot to reveal about themselves if you know where to look. The sidewalls have a string of letters and numbers that some refer to as a code. It’s not a secret, though.

There’s an established method to reading this series of characters. Knowing how to read them and what they mean can help you shop for tires and travel more safely.

Are you ready to solve this mystery? Let’s go!

What Do the Numbers Mean On a Tire? 

To try and break the code, we’ll go by the numbers, starting with a letter or two. See? It may not be apparent already. But don’t worry, we’ll do our best to clear things up.

Vehicle Type

The letter that precedes the first string of numbers indicates the type of vehicle to match the tire. It might be a P, which stands for passenger.

That means the tire is for cars, SUVs, and smaller trucks. Tires with an LT on the side are more rugged for light trucks, vans, and bigger SUVs. Tires specifically for trailers have the mark ST, a special trailer. If you see the letter T, it’s a temporary spare tire.


Immediately following the capital letter or letters, you’ll see three digits that tell you a tire’s width in millimeters. It’s not the overall width, though; it’s what’s known as the “section width.

That’s from the top of the rim, up and over across the thread to the same point on the other side. For example, if you see 205, your section width is 205 millimeters. The larger the number, the wider the tire.

Pro Tip: Proper maintenance is key to keeping your RV tires working well! To learn how to protect them we uncovered Do You Need to Rotate RV Tires Regularly?

Pile of tires in shop.
The inside of your tires can reveal secret info all about their sizing.

Aspect Ratio

The two-digit set of numbers after the slash is the tire’s aspect ratio. It represents the percentage of a tire’s sidewall height concerning its width. If you see a 65 here, for instance, that means the sidewall’s height is 65 percent of its section width.

The higher the number, the taller the tire. This one’s confusing for many people, so we’ll discuss it in more detail shortly.


Next up is another letter that indicates how the tire is made. It’s usually an R, which means it’s a radial tire. Radial tires, the most common kind, have strengthening fibers that run at a 90-degree angle across the tire.

A capital D means the tire has a diagonal ply, and a B means a bias-ply tire. They are all different kinds of tire construction.

Rim Diameter

The following number you see, which is two digits, is the diameter of the rim. The diameter is in inches. In the simplest terms, this is the size of the hole in the center of a tire.

It has to correlate precisely with the size of the wheel or rim. If you see a 15, for example, your rim is 15 inches from side to side.

Load Index

This number indicates how much weight, or load, each tire can bear. It can be two or three digits and is a code within a code. The number doesn’t represent an actual weight but corresponds to a number on a standardized chart.

For example, a load index of 93 equals 1,430 lbs. (or 450 kg.) according to a standard load table.

Speed Rating

This is another code, represented by a letter, which tells you a tire’s safe, top speed. Like the load index, it doesn’t mean anything by itself.

You’ll have to consult a separate conversion chart to determine the tire’s safe speed. The number is on the vehicle at its maximum weight load.

Man shopping for new tires
Make sure you have the right tires for your vehicle by reading the series of numbers inside them.

What Is the Aspect Ratio On Tires?

Aspect ratio – isn’t that something you set on your TV or computer screen? Yes, it is, to make sure the height and width are in sync. The same concept applies to tires. In the 1960s and ‘70s, tires had a much higher profile.

An aspect ratio of 90 percent was the standard for years. With changes in technology and driving habits, today’s tires have a much smaller height concerning their width. They have a lower profile these days. The newer tires handle better because there’s more tread in direct contact with the roads. In general, tires with a taller sidewall may give you a more cushioned ride.

Can I Change the Aspect Ratio On My Tires?

Many of the numbers on the side of your tires are etched in stone. You can’t deviate from the rim diameter, for instance, because another size tire just won’t fit. There is some leeway with aspect ratio, however.

You can choose tires with a higher or lower aspect ratio to improve your performance. You don’t want to mix and match. It’s not a good idea to run tires with different aspect ratios, especially on the same axle.

Pro Tip: Avoid RV tire blowouts with these 7 Tips to Prevent an Awful Accident.

Do Bigger Tires Use More Gas? 

Changing the aspect ratio does affect the overall diameter of the tire. The size of your tires impacts steering, braking, and fuel economy.

Larger tires increase fuel consumption because they have more resistance, and it takes more energy to get them rolling. For the same reason, you’ll get worse gas mileage when your tires are underinflated.

Is It Important to Know What the Numbers On Your Tires Mean?

Using the wrong size tires on your vehicle or having too much or not enough air can cause severe problems. Your ride won’t be as smooth as it could be, and improper tires can affect your steering or traction.

They may also wear out sooner, and you could be in danger of a dreaded blowout. Knowing your numbers (and letters) can help you avoid these kinds of problems. You can make sure your tires are suitable for your vehicle.

Have you checked the numbers on your tires lately? Drop a comment below!

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