Tire Numbers Explained: How to Read Tire Size

Whether it’s your daily driver, off-road rig, street/strip car, or full-out race car, tire selection plays a big role in the performance of your vehicle. But, before you can pick the right tire for your application, you have to understand how to read tire sizes.

When it comes to tires, all the information you need is written on the tire’s sidewall. Tire codes contain key information about the tire, and we’re here to help you decrypt them.

Tire Class | Tire Width | Aspect Ratio | Construction Type | Wheel Diameter | Load Index | Speed Rating

We’ll also go over the three T’s: treadwear, traction, and temperature. Though they aren’t part of the tire number, they’re still important factors when it comes to choosing the best tire for your vehicle and application.

Tire Class

The tire class is indicated at the beginning of the tire code. Typically, you’ll see the letters P, LT, or ST, or no letters at all. These letters (or lack thereof) describe the tire’s class, but also what type of vehicle the tire is best suited for.

No Letter or P = Passenger Car Tire

Tire codes that have a “P” at the beginning are P-metric tires, which means they meet the specific U.S. standards for passenger vehicles. These tires are best suited for cars, smaller SUVs or trucks, minivans, and crossovers.

What If You Don’t See a Letter?

Tires that don’t have a letter at the beginning are Euro-metric tires, also known as metric tires. They’re measured in millimeters, just like P-metric tires, but they have a different load index.

Generally, if a Euro-metric and a P-metric tire have the same size, speed rating, and similar construction, they’re considered equivalent. Just make sure you use them in axle pairs or sets of four. You shouldn’t mix Euro- and P-metric tires on the same axle.

LT = Light Truck Tire

As you may have guessed, tire numbers that start with “LT” are light truck tires. They’re made for larger pickups and work vehicles that tow trailers. Light truck tires are designed for vehicles with a ¾-ton to 1-ton capacity.

ST = Special Trailer Tire

Tire numbers with an “ST” are special trailer tires. They’re used on all types of trailers from utility to fifth wheels.

Tire Width

The three-digit number following the tire class is the tire width. The width is the distance of the tire from sidewall to sidewall in millimeters. For a P185/75 R17 82S tire, the aspect ratio would be 185. This means that the tire’s width is 185mm, or about 7.3 inches.

Aspect Ratio

The next number, which comes after the forward slash, is the tire’s aspect ratio. It’s a percentage that tells you the height of the sidewall compared to the tire’s width. The higher the number, the larger the sidewall. For a P185/75 R17 82S tire, the aspect ratio would be 75, meaning the sidewall's height is 75% of the tire’s width or about 5.5 inches.

Construction Type

The set of letters after the aspect ratio indicates the tire’s construction. For a P185/75 R17 82S tire, the “R” means the tire has a radial construction. Tires may have a bias-ply construction instead, as indicated by the letter “D.”

Bias-ply tires are an older style so they’re more common on classic cars, but they’re also used in drag racing applications. Radials are by far the most common today. If you want to learn more about the differences between the two, check out our in-depth guide.

Sometimes you may see a “Z” speed rating before the construction. The “Z” indicates that the tire can handle speeds of over 149 mph. It’s typically used on high-performance tires. But don’t worry, we’ll get to what speed ratings mean in a second.

Wheel Diameter

The next set of numbers is the wheel diameter or the distance from one end of the wheel to the other. For a tire size of P185/75 R17 82S, the wheel diameter would be 17 inches. Wheel diameter is an important measurement as it determines what size wheel the tire can fit.

Load Index

The last section of numbers refers to the tire’s load index, which is how much weight the tire can handle if inflated properly. Load index charts are an easy way to find out how many pounds the tire is rated for. The load index for a P185/75 R17 82S tire would be “82”, which would equate to a load rating of 1,047 lbs for a four-wheel vehicle when properly inflated.

Most passenger car tires will have a load index between 70-100. Here’s a Tire Load Index Chart to help you find the rating of your tires.

Tire Load Index Chart

[Click to Expand]

Speed Rating

The last letter of the tire code is the tire’s speed rating, which is the max speed the tires are rated for in ideal conditions. For a P185/75 R17 82S tire, it would be “S.” An “S” rating means the tire can handle a top speed of 112 mph.

In some cases, there may be a “Z” speed rating before the construction type. As we mentioned earlier, this is usually the case for high-performance tires. So what’s the point of this extra speed rating? Well, at one point, that was the highest-tested tire out there. But as cars got faster, they needed more letters. So they added more designations for tires that are rated for higher speeds. That’s why the tire speed ratings aren’t quite in alphabetical order.

Treadwear, Traction, & Temperature

Along with the tire code, every tire lists the ratings for treadwear, traction, and temperature. These ratings are determined by the Uniform Tire Quality Grading (UTQG) scale developed by the U.S. DOT.

Treadwear

Treadwear is an estimate of the miles you should get out of your tire before a failure or puncture is likely, but it’s not exact. Higher numbers mean they are rated for more mileage, but that depends on how you use the tire as well. A tire with a treadwear of 400 would have twice as much mileage compared to a 200-rated tire. But if you’re doing a bunch of burnouts, then you can’t expect the tread to last that long.

Traction

Traction is how well a tire can stop on wet pavement. Tires are tested for traction on wet concrete and asphalt skid pads. Traction grades are rated from highest to lowest using a letter scale: AA, A, B, or C. The traction rating depends on the amount of g-Force recorded. For example, AA tires have the most traction and are usually for high-performance applications. They’re rated with a g-Force of over 0.54 on asphalt and 0.38 on concrete.

Temperature

The temperature rating is based on how well the tire can hold up to certain temperatures. It also refers to how well it can dissipate heat and not fail up to a certain max speed. Temperature is rated using a letter scale of A, B, or C:

• A = over 115 mph
• B = 100-115 mph
• C = below 100 mph

Most high-performance tires are going to have an “A” temperature rating, whereas “C” is usually for trailer-style tires.

Of course, everyone wants a tire that lasts a long time and has a lot of traction, but that’s usually not how it works. Treadwear numbers go down as traction goes up, so you will always sacrifice one over the other. The softer the tire, the better the traction. But they’ll also wear faster. That’s why drag radials have a treadwear of “0.” It’s a performance tire that doesn’t guarantee mileage.

Tire Compounds

Tire compounds are usually made of a combination of natural and synthetic rubbers. Natural rubber is designed for wear and tear, while synthetic rubber is going to help with the tire’s performance. Along with other chemicals and antioxidants, there are all kinds of things that make up a tire’s compound. The compound determines whether it’s a harder tire for a heavy-duty truck or a softer tire for performance.

The specific chemical makeup of each tire will vary from brand to brand. Usually, the best-rated tires use the best compounds. When it comes to finding the best tires, it’s usually best to go with a brand that has a good reputation for both reliability and performance.

Tire Tread Patterns

Another thing you want to consider when choosing your next set of tires is the tread pattern. This is especially important if you plan on doing any type of performance driving. There are three different types of tire tread patterns: symmetrical, asymmetrical, and directional.

Symmetrical

On symmetrical tires, the tread is the same on the inner and outer sides of the tire. This means the tread and grooves are a mirror image on each side. They can be rotated side-to-side and front-to-back so you can get more life out of the tire.

They’re one of the most common tread patterns and are often seen on passenger cars because of their longevity, versatility, and quiet ride.

Asymmetrical

Asymmetrical tires have a completely different tread pattern from side to side. These tires are typically included on sports cars. The variation in tread makes them suitable for many applications, but especially cornering. The inner and outer sidewalls are marked as they have to be mounted a certain way.

Asymmetrical tires have excellent ride quality and offer better performance than symmetrical ones.

Directional

Directional tires have a tread pattern that’s designed to roll in one direction. Because of this, they can only be rotated from front to back. But they’re excellent for dispelling water, which reduces the chances of hydroplaning.

Directional tires offer good ride quality and performance as well as great traction in many different weather conditions.

How to Pick the Best Tire for Your Vehicle

There’s a lot more to tire selection than simply picking the cheapest tire that fits your wheel. In addition to the considerations above, you have to take vehicle fitment into account as well. In other words, how the tire fits your specific wheels, vehicle height, driving applications, and more. As a general rule, you shouldn’t go more than 20% wider than the wheel. So if you have a 10-inch wide wheel, a 12-inch wide tire is about as wide as you want to go. If you end up purchasing white letter tires, check out our guide on how to remove the blue coloring from your new tires.

For more help choosing the best tires for your vehicle, check out some of our tire buying guides. And for updates and deals on the latest tires and parts for your Ford, Jeep, or Chevy, be sure to sign up for our newsletter.

Source: How to Decode Tire Size and Other Data, Consumer Reports