How To Read Tire Sizes

Tire sizes are a little tougher to read than t-shirt sizes, but not by much. Let us show you how.

byChris Teague|
how to read tire sizes
Peter Nelson
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Nobody truly enjoys spending money on new tires. They’re expensive, wear more quickly than we’d like, and can be a real pain to determine the right kind. Those may come to mind when buying new rubber, but have you actually taken the time to understand what the sizes and markings on your tires mean?

For better or worse, we live in a world where litigation is king and where companies are careful to list every possible warning and bit of information that they can on a product. Tires are absolutely no exception, but the sizing figures you’ll find on the side of your new Michelins are a bit more complicated than just a number or letter.

The sizing information on your tires can tell you many things, not just the size. The letters and numbers tell you how fast you can drive, how heavy a load the tires can handle, and can even give you an idea of how comfortable those tires will be to live with on a daily basis. Let’s get into tire sizes and why they’re important.

Warning: The Drive’s editors are going to ask you to do some math in this post. We’re sorry.

Why Do I Need To Know The Size Tire That Comes On My Car? 

Well, for starters, we assume you want to get the right size tire when you have to pony up the cash? Your local tire shop can look up the tires that came on your car, but what if you bought an options package with a special wheel size? What if the person that owned the car before you put on custom wheels? 

What Do Speed Ratings Mean, And Why Do They Matter?

A tire’s speed rating is the speed at which it can safely carry a load. Different types of tires carry different speed ratings. For instance, an S-rated tire can handle 112 mph, while a Y-rated tire can safely handle up to 186 mph.

Common Speed Ratings (miles per hour represents the maximum safe speed for each rating):

  • S: 112 mph
  • T: 118 mph
  • U: 124 mph
  • H: 130 mph
  • V: 149 mph
  • Z: >149 mph
  • W: 168 mph
  • Y: 186 mph

You might be asking why Z-rated tires don’t hold the title for the fastest tire. That’s because when Z-rated tires were introduced, it was common thinking that 149 mph was the fastest a tire could ever safely be rated. Now, we know better, which is why there are a total of three ratings on top of Z’s former top spot. Above Y-rated tires are (Y)-rated tires, which can travel more than 186 mph. 

Here’s How To Read Tire Sizes And Change Tire-Wheel Sizes 

Let’s do this! 

Reading Tire Sizes

  1. Locate the tire’s sidewall, which is the portion between the wheel and the tread.
  2. You will see several notations on the sidewall, including a brand name and a model name.
  3. The tires’ size will be prominently listed on the sidewall. This is a string of letters and numbers that usually starts with “P.” For this example, we’ll use P215/55R17 tires, found on a 2019 Toyota Camry Hybrid.
    • “P” refers to the fact that a tire is P-Metric, which means it meets standards put in place in the United States for passenger vehicle tires.
    • The number immediately after that, 215 in this case, indicates the tire’s width. This tire is 215 millimeters wide.
    • The aspect ratio is listed directly after the slash. These tires have an aspect ratio of 55, which means that the tire’s height is 55 percent of its width. The larger this number, the “taller” a tire is.
    • The “R” here stands for radial, which indicates that the layers are configured radially across the tire.
    • The last number here is 17, which is a measure of the wheel diameter.
    • Many tires will feature another number at the end of the string, accompanied by a letter. This indicates the load index and speed rating.

That’s it! You did it!

Changing Tire Sizes

We know that you may get bored with your ride’s rubber, did you know you can size them up or down? Let’s do this.

  1. To change wheel size, you’ll need to determine the overall diameter of the wheel and tire. This is important to keep your vehicle’s on-board measurement tools, such as the speedometer, accurate.
    • To do this properly, you’ll need to multiply the tire’s aspect ratio by two, which will give you the overall width that the tire adds to the wheel.
    • Convert to inches and add this number to the wheel’s diameter. This will give you the entire rolling diameter of your current tire and wheel setup.
    • You’ll need to do the same calculations to find out how big or small you want the new wheel to be. The new wheel and tire diameter will need to be the same as the old tire and wheel diameter.
You don't have to rely on experts to read tire sizes.

Tire Size Terms You Need to Know

Tire Types

We won’t get too deep into this topic in this post, but it’s important to understand that tire size is just one factor when it comes to picking out the right rubber for your car. Your vehicle likely left the factory with all-season tires, which are meant to work in a wide variety of temperatures and weather conditions, but not all.

Dedicated summer and winter tires are beneficial for people who live and drive in warm and cold climates, respectively. Make sure that the type of tire you pick is aligned with where you live and drive regularly.

Wheel Sizing

Many of today’s wheel sizes are larger and more complex than even the most expensive and exclusive custom wheels from just ten years ago. Some automakers are moving up to 22-inches and above for SUV and pickup truck tires, which would have been unheard of outside of a few over-the-top customs debuting at SEMA.

Drive Wheels

If your vehicle is front-, rear-, or all-wheel drive, you’ll be faced with different requirements on rotating and buying new tires. Depending on which wheels are being driven on your vehicle, you may have to buy all four tires at once, or you might be able to get away with just two at a time.

Sidewall

People probably spend a lot less time thinking about their tires’ sidewalls now that whitewalls aren’t really a thing anymore, but they’re an important part of understanding a tire’s size. The sidewall is the vertical portion of a tire that starts where the rubber connects to the wheel and ends where the tire tread begins.

Tread

Tire treads are the part of the rubber that actually meets the road. You’ll need to understand how wide the tire treads are in order to accurately measure the tire’s size. Treads also gain importance when we’re talking about different types of tires, as the pattern and arrangement of treads vary for each specific tire.

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