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.
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.
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.
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.
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
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.
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
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]
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. Like 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.
Tire Speed Ratings
||Maximum Speed (mph)
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 is an estimate of the miles you should get out of your tire, 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 is how well a tire can stop on wet pavement. Tires are tested for traction on wet concrete and asphalt
skidpads. 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.
Tire Traction Ratings
||Less than 0.38
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, where “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 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.
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
They’re one of the most common tread patterns and often seen on passenger cars because of their longevity,
versatility, and quiet ride.
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 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
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.
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.
Sources: Virginia Tire & Auto, Consumer Reports, Tire Buyer