Tire Specs: Understanding the Numbers on Your Tires

If you’ve ever looked over the numbers and letters stamped into the sidewall of your tires, you’ll know there’s a lot of information available in a small amount of space. But what does it all mean? Following is a guide to help you understand that information so that you’ll have an easier time determining which tires are right for your vehicle.

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Are Tire Specs Standard?

Fortunately, the answer to this question is yes. Here, a standard set of interchangeability guidelines is governed by The Tire and Rim Association for all manufacturers of tires in the United States. They also have established relationships with most major tire manufacturers in Japan, Europe, South America, and other parts of Asia.

Overview

Tire Size Specifications

Tire-Size-Specifications

It’s important to understand that tires come in many different sizes that can determine the tire’s performance, their suitability for your specific vehicle, and their overall appropriateness for your driving needs. It is recommended that you check your owner’s manual to understand what size of tires are correct for your vehicle. The following information about your tire’s size and type may be found on its sidewall:

1.    Type of Tire

Type-of-Tire

The letter or letters at the beginning (or end) of this sequence indicates the type of tire: P, LT, ST, or T. Most tires will have the letter P, which is short for “P-metric” or a passenger car type tire. These are typically installed on passenger cars, light trucks, SUVs, and vans. An LT appearing at either the beginning or the end of the sequence indicates the tire is a “light truck” tire, or a tire designed to be used on vehicles that carry high loads or pull heavy trailers. ST indicates a “special trailer” tire, used on (you guessed it) - trailers, while “T” denotes a “temporary” tire, like a “donut spare” you might find in the trunk of your vehicle.

2.    Tire Width

Tire-Width

The first set of numbers in the numerical sequence indicates the width of your tire as measured in millimeters from sidewall to sidewall.

3.    Aspect Ratio

Aspect-Ratio

Next you will find a slash and then another set of numbers: the aspect ratio, sometimes called the tire’s “profile”. This number is a percentage that indicates the tire’s height vs. its width. In the example above, the aspect ratio of our tire is 65, meaning that this tire’s height is 65% of its width.

While there are exceptions, a lower aspect ratio is often indicative of a sportier, more performance-oriented tire while a higher aspect ratio tends to indicate the tire was designed for other priorities like comfort or rugged durability.

4.    Construction Type

Construction-Type

Next you will find a single letter, either a “D”, a “B”, or an “R” which indicates the construction type for the tire. “R” indicates that the tire is a Radial tire and “D” designates that it is a Diagonal (also known as “Bias”) ply tire and “B” means it is a Belted-Bias ply tire .

Today, tires featuring Radial construction are by far the most common type. They are made with layers of fabric cords positioned at a 90’ angle to the center line of the tread. Radials became the dominant type of construction because of their superior fuel economy, traction, ride comfort and especially tread life when compared to earlier types of construction. Diagonal (or Bias) ply tires employ layers of cord which cross each other at a 30’-45’ angle to the center line of the tread. This type of construction dates back to the earliest pneumatic tires.  It was the preferred design for many years because of its resistance to damage from poorly paved or unpaved roads and relative ease of construction on the building machines available a century ago. Belted Bias tires combine the diagonal ply casing of Bias tires with belts (usually made of fiberglass) under the tread for increased traction and tread wear vs conventional Bias ply tires.  Both Bias and Belted-Bias ply tires are available today primarily as reproduction tires for antique vehicles, though some racing, trailer and off road light truck tires use these types of construction, too.

5.    Rim Diameter

Rim-Diameter

The next number in the set indicates the rim diameter—sometimes also noted as the wheel diameter. This measurement is almost always given in inches. Though there are exceptions, rim diameters ranging from 13” to 22” are most common for passenger vehicles in North America.

Other Information Found on the Tire Sidewall

1.    Load Index

The next number, which is paired with a letter, indicates the load index for that tire. The load index is the maximum amount of weight that an individual tire can carry. Because load indexes for a given tire size may vary, it is important to check to be sure the tire you’re considering meets or exceeds your specific vehicle’s manufacturer’s requirements.

load-index

2.    Speed Rating

In Europe, some highways have speed limits higher than those in North America – and a few have no limits at all.  In order to give drivers and government authorities an indication of a tire’s ability to withstand the stresses associated with elevated speeds, the speed rating system was developed.  Simply put, the speed rating is the maximum speed at which a set of properly inflated tires may be driven. In North America, speed ratings are useful mostly as an indicator of a tire’s performance capability – how responsive is its handling, how strong is its braking traction, etc.  Since there can be large differences in handling and traction for tires within a given speed category, speed ratings alone shouldn’t be used to determine tire choice. You can find more information about tire speed ratings here.

tire-speed-ratings-chart

3.    Treadwear (UTQG), Traction, and Temperature Grades

Close to where the sidewall meets the tread, you will also find the Treadwear, Traction, and Temperature grades. These grades are assigned by each tire maker to its own products.  They may be used to make comparisons within a tire maker’s product line, but because of variations in testing procedures, they should not be used to compare tires from one company with those made by another company.

4.    US Department of Transportation Code

Every tire sold in the United States is required to include a US Department of Transportation (US DOT) Tire Identification Number on at least one sidewall.  It is located near the rim, and it starts with the letters DOT. These letters are followed by a sequence of 8-13 letters and numbers that denote that tire’s manufacturing location, its specific size, and the manufacturer’s identifying code. The final four digits indicate the week of manufacture followed by the last two digits of the year of manufacture.  For example, 5014 would indicate a tire manufactured during the 50th week of 2014. On the sidewall opposite the one with the full DOT code, only the letters DOT and the first four digits of the code are required.

5.    Tire Ply Composition and Materials

Information on the materials used in the tire’s internal construction is listed next to the DOT code. This information includes both the ply composition—that is, the number of layers as well as the type of materials that were used in its construction.

6.    Maximum Load and Cold Tire Inflation

Finally, in the area of the sidewall closest to the wheel, you’ll find the tire’s load capacity and its maximum cold inflation pressure. This might be different from the inflation pressure recommended for your vehicle by its manufacturer. That’s because the maximum cold tire inflation pressure stated on the sidewall only indicates the greatest pressure permitted for that tire.  It doesn’t state the optimal pressure for a tire mounted on a particular vehicle – that is found on a placard mounted on the vehicle or in its owner’s manual.

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Bridgestone - Tire Tread Patterns

You probably know what tire tread is: the part of the tire that makes contact with the road surface. But what you might not know is that the specific tire tread pattern your tire utilizes can impact how your tires perform, such as how they handle and how much traction they provide in different driving conditions. Let’s look deeper at both the elements of tire tread patterns as well as the three types you might run across in your search for new tires: Symmetric tread, asymmetric tread, and directional tread.

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Symmetric Tread Pattern

The most common tread pattern in use on passenger tires is the symmetric tread pattern. In these types of patterns, the outer and inner sides of the tread are symmetric—that is, they are mirror images of one another. The tread blocks form continuous ribs around the tire. Tires with a symmetric tread may use any type of rotation method – front-to-back, modified X or full X. An example of a symmetric tread pattern is the Bridgestone Ecopia EP422 Plus. At times, you might hear symmetric tread pattern tires referred to as “multi-directional”. This is because the tire can be mounted on the wheel and rotated in any direction without affecting performance.

Asymmetric Tread Pattern

Another tread type found on passenger tires is the asymmetric pattern. In these types of patterns, the inner and outer halves of the tread are designed with different channels and tread blocks. This design is intended to enhance performance such as water dispersal or dry grip and snow traction. In the example above, the Bridgestone Turanza Serenity Plus all-season touring tire, you can see how the left (outer) side of the tire tread features chunkier tread blocks that provide a larger contact area for increased stability and handling on dry roads. The right (inner) side of the tread pattern features more channels and smaller tread blocks to aid in channeling water away for improved rain traction. Multiple rotation patterns may be used for this type of tire.

Directional Tread Patterns

Some tire tread patterns are designed for rolling in a unidirectional way, meaning in only one direction. These are called directional tread patterns and are often found in high-performance tires and winter tires. As you can see in the example above, the Potenza RE960AS Pole Position tire features tread elements that form a V-shape. This V-shaped tread pattern helps move water more efficiently from under the contact patch to help boost hydroplaning resistance at higher speeds. Since the channels only work if mounted with the V’s oriented correctly, once the tire is mounted, directional tires can only be rotated using a front-to-back and back-to-front pattern. Directional tires will feature an arrow on the sidewall indicating which direction they need to roll when mounted on the car.

 

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