Understanding how to decipher all of the numbers and letters on a tire’s sidewallplays an important part in making sure you choose the right tires for your vehicle. The first bit of information you’ll need to decode is tire size.
What does tire size mean?
Tire size could also be considered “tire specifications”. Because that’s what it is — the term tire size is in reference to the long sequence of letters and numbers that’s branded on the sidewall of each of your tires. In this chain of numbers lies information about your tire’s class, dimensions, speed rating and load range— more broadly known as the specs for your specific tires.
So, sure, tire size does include physical dimensions, but it also includes a whole lot more information that — and, with a little practice, you’ll be able to decode it in no time.
Decoding tire size: Class
Let’s take it from the top. Most tire sizes — the strings of letters and numbers that encircle your tires’ sidewalls — might begin with a single letter or multiple letters that indicate the tires’ class or intended purpose. Were your tires made to carry the kids to school and you to work? Or were they made to haul huge loads across the country?
P — Meaning P-Metric. If there is no “P” in front of the size designation, it is known as a European- Metric or Hard-Metric. In the past, P-Metrics were the most common size designation found on domestic cars, minivans, light trucks and SUV’s. Foreign cars generally came with European Metric tires. These days however, you may find either designation on any make or model.
LT — If they were made to carry heavier loads, such as heavy-duty pickup trucks and SUVs, your tire size would begin with an “LT.”
ST — You may also encounter an “ST,” which means that the tires are designed for special trailer service and are meant to be used on heavy trucks and trailers only.
C — A less common but still relevant letter you’ll see at the start of some tire sizes is a “C,” which means that the tire has been made for commercial vehicles, such as vans and delivery trucks that are capable of carrying heavier loads.
T — If you come across a tire size that begins with a “T,” you’ll know that it’s a temporary or mini spare, sometimes called a space saver or a donut. As you probably know, this type of tire should be used only in the event of a flat tire.
Decoding tire size: Dimensions
The next set of information you’ll encounter when trying to decode your tire size — the string of letters and numbers that encircle your tire’s sidewall — includes three different groups of numbers and letters that designate dimensions.
First, you’ll find a three-digit set of numbers that identifies your tire or section width. Following that, you’ll find a slash and two numbers that detail your tire’s aspect ratio. Next, you’ll see a letter that indicates internal construction. And the final part that goes into the dimensions part of the tire size is the two-digit number that identifies wheeldiameter.
Tire or section width — The first three numerals in your tire size equal your tire or section width in millimeters. The tire or section width is measured from the widest part of the tire’s outer sidewall to the widest point of its inner sidewall excluding raised letters or decorations.
Aspect ratio — Following the slash in your tire size, you’ll come across two numbers that indicate your tire’s aspect ratio. The aspect ratio indicates what percentage your tire’s sidewall height is in relation to its section width.
Internal construction — After the two-digit aspect ratio, you’ll come across a letter, which signifies the tire’s internal construction method. For example, if you see an “R,” it means that your tire has a radial construction (the most popular type of tire). A “D” typically means that the tire’s internal plies crisscross in a diagonal pattern with bias ply construction. And, if you see a “B,” it means that your tire has belted construction (uncommon).
Wheel diameter — Next, after the single letter, you’ll find a two-digit number that indicates the diameter of the wheel in inches.
Decoding tire size: Load index
Your tire load index, also known as the load carrying capacity, is designated by the two or 3 digits near the end of your tire size that precede the single letter.
For example, if you see “92H” at the end of the tire size, the load index would be 92. The higher the tire load index, the more weight the tire can safely carry. In the case of a European Metric tire with a 92 load index, that tire would be able to support 1,389 pounds at maximum air pressure. When you multiply that amount for a single tire by four, your vehicle’s maximum load capacity would be 5,556 pounds.
Decoding tire size: Speed rating
Typically found last is the letter that indicates the speed rating, also known as the maximum service speed. The simplest way to define speed rating is the fastest speed a tire can handle before it no longer performs as designed. It should not be considered the maximum speed that you can safely drive — especially in adverse weather or road conditions. Learn more about speed rating and see a speed rating chart here.
Decoding tire size: Load range
Your tires might or might not have a load range listed on the sidewall. If there is one, it will typically be the last letter or set of letters at the end of your tire size sequence. Load range, sometimes known as ply rating, is a term that defines the tire’s maximum allowable air pressure and how much weight it can — or should — carry, per industry standards. Learn more about load range here.
SL — Your tires are in the standard load range if they have either no letters or an “SL” at the end of their tire size.
LL — Your tires are in the light load range if they are branded with an “LL” at the end of their tire size.
XL or RF — Your tires are in the extra load or reinforced load range if they include either an “XL” or an “RF” in their descriptions.
B, C, D, E or F — If you drive a light truck, your tires have likely been manufactured in one of three load ranges: LT-metric, LT-flotation and LT-numeric. Light truck tires typically have load ranges of B, C, D, E or F — all of which have different, corresponding ply ratings — 4-ply rated, 6-ply rated, 8-ply rated, 10-ply rated or 12-ply rated, respectively.
B, C, D or E — If you’re driving a heavy truck that requires special trailer service tires, those tires are going to be ST-metric sized. Special trailer service tires have load ranges of B, C, D or E, which correspond to their specific ply ratings — 4-ply rated, 6-ply rated, 8-ply rated or 10-ply rated.
How to find tire size
If, for some reason, you’re unable to clearly read the tire size on the sidewall of your tire, you can also usually locate this information inside your glove compartment, your gas tank hatch and your driver’s side door. A tire professional will always be willing to assist you as well.
Vehicles do not always come with a spare tire, but you might not notice this until it’s too late (when you are on the side of the road with a flat). It’s important to always have a back-up plan. Tireamerica.com has a guide created to help you understand the best options for you.