Wondering what tire size means? We can help. Read on as we explore the meaning of tire size to uncover its definition, origin, and more.
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Kevin
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March 2, 2022
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Common Questions
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When you’re on the hunt for tires, you may come across some auto service terms that you’re unfamiliar with, such as tire size. Seeing as the tires on your car are imperative to get you from point A to point B safely, having a basic understanding of this term is of the utmost importance for drivers.
In this guide, we explore the automotive industry to uncover everything you should know about tire size, including its definition, importance, and more. So buckle up, and keep reading!
What Is the Definition of Tire Size?
Tire size can be defined as a collection of numbers and letters that describe tire type, width, sidewall height, and wheel diameter — alongside other information. An example of a common tire size is as follows: P225/65R17.
The tire size for your vehicle and wheels can be found in two places:
- The sidewall of your tire
- Inside the frame of the driver side door
- Within your gas tank hatch
How Can I Read Tire Size Numbers?
Understanding how to read tire size can be a little confusing— especially if you’re not too well-versed in automotive specs.
To help, let’s break down the tire size example that we provided above (P225/65R17):
- Tire type: Your tire will begin with a letter explaining what type of tire it is. The letter “P” refers to a Passenger car tire. If there’s no letter before the first number, you likely have a metric tire most commonly referred to as European size (E-metric).
- Tire width: The next number — such as 255 — is the width of your tire, measured in millimeters, from sidewall to sidewall. This determines the width of the rim (aka rim diameter) where your tires are mounted.
- Aspect ratio: Following the tire width, you’ll find the aspect ratio (65), which refers to the height of the sidewall as a percentage of the width.
- Construction type: The letter “R” indicates that this is a Radial tire, designed for fuel efficiency and strength.
- Tire diameter: The number 19 that comes after the letter “R” refers to the inside diameter of the tire in inches. Tire diameter determines the size of the wheel where the tire is mounted.
All tires sold in the United States must meet the standards agreed upon by the Tire and Rim Association (TRA) and the European Tire and Rim Technical Organization (ETRTO) — these standards are virtually interchangeable.
What Is the Origin of Tire?
In 1846, a Scottish entrepreneur named Robert William Thomson filed a patent in France for a wheel that he called “leather filled with air.” This was the first tire known to man.
Fast forward to about 40 years later, a veterinarian named John Boyd Dunlop invented a different kind of tire. On December 7th, 1888, he patented his revolutionary invention, calling it the pneumatic tire.
In the following years, Dunlop’s tire underwent a range of developments, both by the inventor himself and by others (such as Thomas Hancock), ultimately paving the way for the versatile tires used today.
Automotive Terms Related to Tire Size
Now that you understand what tire size means let’s take a look at a few other automotive terms that are related to tire size. Discovering like-words won’t only help to further your understanding of the term tire size, but it can improve your overall vocabulary, as well.
- Load index indicates the maximum load that the tire can support when inflated.
- Speed rating tells you the maximum speed capacity of a tire.
- DOT symbol: indicates that the tire complies with all the applicable Federal Motor Vehicle Safety Standards in the U.S.
- Tire identification number is the series of numbers and letters following the DOT symbol.
- P-class tire or passenger vehicle tire is represented by the letter “P” and includes cars, SUVs, crossovers, minivans, and smaller pickup trucks.
- Light truck tire or LT is a tire type designed for vehicles that are capable of carrying heavy cargo, towing trailers, or for those looking for an extra heavy-duty option.
- Special trailer or ST refers to tire sizes that are meant for trailers.
- P-metric is used to identify a tire that is primarily intended for passenger cards.
- Load range can be defined as an alphabetic code that designates for a specific tire size the max load a tire can carry at a defined inflation pressure.
- Rotation refers to the moving of tires from corner to corner on a vehicle according to a set pattern.
- Traction grades represent the tire’s ability to stop on wet pavement.
- Uniform Tire Quality Grading or UTQG is a system developed by the Department of Transportation (DOT), to provide shoppers with information to help them purchase tires based on their relative treadwear, temperament capabilities, and traction.
To Recap
Sizing information provides a significant amount of detail about the tire, such as the:
- Tire type
- Tire width
- Aspect ratio
- Tire’s construction type
- Wheel diameter
- Load index
- Speed rating
Learning how to read tire size can be confusing, but it’s important. That being said, if you ever feel stuck or unsure, feel free to refer back to this guide. Whether you’re shopping for a new set of tires, looking to replace one broken tire, or simply curious about the numbers etched into your tires, you can always count on us to help!
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