Decoding Tire Size: Understanding the Numbers

Updated: November 26, 2025 by Michael Kahn. Published: November 26, 2025.

Whether you’re replacing a damaged tire or purchasing a new set of tires, it’s important to know what tire size is the right fit for your vehicle. A tire with the wrong size either won’t be able to be mounted on your vehicle or it will result in a bad performance, which could result in damaging other components. Of course, nobody wants to experience this, which is why having the knowledge of tire size matters.

Besides avoiding the mentioned issues, understanding the tire size is also beneficial since it’ll help you better understand your car and its performance. Since there’s a large variety of both tires and vehicles, it’s helpful to acquaint yourself with their capabilities to get the most out of them while remaining safe. If this sounds good, let’s decipher the tire size without any further ado!

The Secret Code: Decoding the Primary Sizing

The first thing you come across when reading the tire size is a sequence of letters and numbers, which may look something like this on the tire’s sidewall: 205/55R16. If it looks confusing, don’t worry, we’ll take it slow and analyze it bit by bit.

The first number, 205, is the tire width. It’s measured from sidewall to sidewall across the footprint and is expressed in millimeters. This means that a tire with the 205/55R16 size has a footprint that is 205 millimeters wide.

The number after the forward slash is the aspect ratio, which in our case is 55. The aspect ratio is the ratio of the height of the tire’s sidewall, measured from from wheel rim to the top of the tread, to its width, which is shown in percentages. In our example, the aspect ratio is 55% of the tire’s width. The bigger the aspect ratio, the higher or taller the tire’s sidewall or profile is.

The letter following the aspect ratio marks the tire’s construction. “R” indicates a radial construction, while “D” signifies that a tire has a diagonal structure. In other words, these letters indicate how the tire’s plies are organized within the tire, which allows tires with one structure to perform better in one set of conditions, while tires with another structure will provide better performance in another set of conditions.

The last number in the tire size is the wheel diameter. It measures the distance between the two bead seat areas and is expressed in inches, which, for our tire size 205/55R16, means that there’s 16 inches from one side of the wheel to the other. In simpler terms, this number tells the rim size the tire is designed to fit.

Speed & Load: Beyond Just Size Longevity

Besides tire width, aspect ratio, construction, and rim size, the tire size also includes the speed and load ratings. These indexes come after the mentioned notions, so you may see something like this on the tire’s sidewall: 205/55R16 91V.

The number 91 in our example is the load index. It can range from 1 to 150, and it tells drivers how much weight a tire can handle, which means a higher number enables the tire to withstand more. A load index between 75 and 100 is typical of passenger cars, while a load index of SUVs is above 100. Metric light truck tire sizes will have two load ratings (107/103). The load index 91 means a tire can withstand 1356 lbs when inflated to its maximum.

The letter accompanying the load rating is the speed rating, which in our example is the letter V. Similarly to the previous case, the speed rating represents the top speed a tire can safely maintain for a long period of time. The symbols range from A1 to ZR in an alphabetical order with the exception of the letter H. The speed rating is determined based on the tire’s performance category and the capabilities it needs to provide. In simpler terms,some tires are designed for high speeds while others are not, so one shouldn’t expect that from every tire.

Decoding the “Extras”: Service Type & M+S

While we’ve covered most of the tire size, you can still find a few other details on a tire’s sidewall. One such thing is a service type, which is essentially a letter telling us the type of vehicle the tire is designed for. It is found at the beginning of the tire size: P 205/55R16 91V.

“P” indicates that a tire is for passenger vehicles, including regular cars, SUVs, crossovers, minivans, and electric vehicles.

“LT” signifies a light truck tire, which is created to withstand heavy loads and is also good for towing trailers.

“ST” is a letter combination that means special trailer, encompassing different kinds of trailers like boat and utility trailers, to mention a few.

In addition to this, you will also stumble upon the M+S symbol on a tire’s sidewall. It stands for mud and snow, indicating that the tire can be used on muddy and snow covered roads due to the geometry of the tread design. However, only the tires with the three peak mountain snowflake (3PMS) symbol should be used for winter, as they meet the industry’s requirements.

Now, after discussing the tire size in detail, it’s not that difficult to read tires, or learn why your tires are turning brown, right?

Conclusion

There are many different aspects of tires to consider when you’re thinking about replacing your existing tires. Whether you need a model that can handle high speeds, heavy-duty performance, or is intended for a particular vehicle type, the tire size is there to help you see if the tire you chose is the right fit for you. Of course, don’t forget to check the vehicle’s owner’s manual or the sticker on the driver-side door jamb to see what tires would be the most suitable for your car. Following these specifications will help you find the perfect match for your vehicle.