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How to Read Tire Specs (In-Depth Explanation)

If you look at the sidewall of your tires, you will notice numbers and letters printed on them. Have you ever asked yourself what do these numbers and letters indicate? Browse this article to understand how to read and interpret the information on the sidewall of your tires. We have got a lot to cover, from aspect ratios, load index to the manufacturing date. So let’s get started.

Tire Size Description

The tire size is by far the most significant piece of information printed on a sidewall. Let's have a look at how to read it correctly first.

The size of the tire will be represented as follows in most cases: P225/40/R17

Note: While the numbers will differ from tire to tire, the formatting will remain the same. Let's take a closer look at this size classification (which is the manufacturer size for a 2012-2015 BMW 328i, if you were wondering).

The letter “P.”

The letter "P," which comes before the remainder of the size indication, expresses the first significant piece of information.

The letter "P" stands for the passenger in this case, signifying that the tire is designed for use on passenger vehicles.

Sedans, crossovers, coupes, and smaller SUVs would be considered passenger vehicles.

The letters “LT” for light truck, which covers larger SUVs and pickup vehicles, may be used for the letter “P.”

A “T” for transitory may also be seen, signifying the usage of a spare or interim tire.

In some circumstances, you may come across tires with nothing before the first digit in the tire size label.

These tires are known as Euro-Metric, contrary to their P-Metric equivalents described previously.

Euro-Metric rubber and P-Metric rubber have slightly differing load ratings and inflation pressure but are generally regarded equivalent.

The numerals: 225/40/R17

The basic tire size information follows the "P."

The tire's diameter is represented by the first digit in this series, in this case, 225. The tire area that makes touch with the ground is the precise width of this initial number.

This measurement is in millimeters; therefore, the tire shown is 225 mm wide. The following number, in this case, 40, is the aspect ratio, which measures how wide the sidewall is compared to the tire's width.

The aspect ratio of rubber is the proportion of the width that the sidewall occupies.

In the illustration above, the sidewall of this tire is accurately 40 percent of the width, which is 225mm. As a result, 40% of 225 equals 90, resulting in a 90mm sidewall.

The final number and character in our size description tell us about the tire's composition and the width of the tire's center circle. The bead of the tire is what it's called.

Let's begin with the letter R, which denotes that this tire is made of radial material rather than non-radial or bias-ply material. Radial tires contain independent tread and sidewall plies of rubber.

On practically all single-passenger cars, this style of tire is the accepted standard. Other characters may denote extra features of the tire, although it is uncommon to see anything other than an R on most tires.

The tire's rim width is measured in inches after the R, in this scenario 17. This means the tire will fit on a 17-inch wheel.

Service description

The service description is the next piece of information on the tire wall. A two- or three-digit number followed by a single letter, such as 89Y, will represent this letter and number combination.

The service description is often used to represent two things: the tire's load index and the tire's speed rating.

Load index

In our previous example, the load index is described in the service description's numeric component (89). This is the highest amount of weight that a single tire can safely support. The load index number does not accurately reflect the actual weight.

To match this figure to a respective weight, you'll need to examine a load index chart. A load index of 89, for example, equates to a tire designed to support 1,279 lbs.

Speed rating

The tire's speed rating is indicated by the letter that accompanies the load index element of the service description. As the name implies, the speed rating is the highest speed at which the tire may safely navigate.

Like the load index, the letter does not reflect a speed; however, it is used to reference a chart that will show you the actual speed rating. The 89Y tire, for example, can withstand speeds of up to 186 miles per hour.

Department of Transportation (DOT) Code

The DOT code is the next critical piece of information you'll find on the sidewall of your tire. The DOT code is an 8 to 13-digit number that begins with the letters DOT. It will be presented in a smaller font and less prominent than the prior two items we discussed.

In its statement of four separate things: tire size, tire maker, factory location, and date of manufacture, the DOT code is a little redundant. The following is an example of a DOT code: DOT 4B08 4DHR 4116.

The presence of the initials DOT in the above code indicates that the tire on which they are written fulfills all DOT tire specifications and complies with all DOT regulations.

In our illustration, the following two sequences of numbers, 4B08 4DHR, give us the tire's size, who made it, and where it was made. This detail is unnecessary because we already know the size and brand from the format we covered earlier.

The location of the production factory is the only useful piece of information we have. If you're looking for a tire made in the United States, search for specific codes on the tire.

The last four digits of a DOT code tell us how old a tire is or when it was made. The first two numbers indicate the week in which the tire was made, while the last two numbers indicate the year.

So, our sample tire, which has a DOT code that finishes in 4116, was made in the 41st week of 2016.

Uniform Tire Quality Grade (UTQG) Standards

The tire's Uniform Tire Quality Grade is the final significant piece of information on the sidewall (UTQG). The DOT created the UTQG metric in an attempt to provide consumers with a better knowledge of a tire's capabilities while purchasing.

As a result, the UTQG specifies the tire's treadwear lifetime and its traction and temperature resistance. The UTQG rating is represented by a three-digit number accompanied by 2 to 3 letters, such as 300 AA A.

Reading Treadwear

The treadwear grade of the rubber is the first piece of information presented in a UTQG code. The treadwear rating indicates how long the tire should last or how rapidly it will wear out.

While this may appear important information on the appearance, the method by which treadwear rating is calculated renders it almost meaningless when evaluating tires from different brands.

Let's take a closer look at how tire companies determine treadwear ratings to see why.

Treadwear ratings

All treadwear ratings are computed after a tire producer has put its tires through a regulatory test process. To begin accumulating data, tire testing begins on a 7,000-mile track.

After completing the course, the tire company will analyze the tire, determine how long it will last, and award a treadwear rating. We can guess how long the tested tire is anticipated to survive compared to the control tire based on our case of 300.

So, if our trial tire was given a treadwear value of 300, it meant that it was expected to survive three times as long as the control tire it was compared to.

It was expected to last 4 times longer than the trial tire if given a treadwear rating of 400, 5 times longer if it was given a treadwear value of 500, and so on. If it isn't evident, treadwear rating isn't a handy statistic for a variety of reasons. Each tire maker will use a separate control tire to match the test tire. As a result, the treadwear scale varies significantly from one maker to the next.

Furthermore, the treadwear rating is determined after a manufacturer's employee inspects the tire. Nothing is stopping them from being as enthusiastic as they want to be about tire longevity estimates.

This implies they can artificially raise the treadwear rating without risking severe consequences. To summarize, treadwear rating is a statistic born of a shoddy testing technique and should be treated with a pinch of salt the size of a boulder. Evaluating tires from the same brand can be helpful in some circumstances. When considering brands, however, it isn't very sensible.

Traction grades

The traction grade is the 1 to 2 letters following the first and sole digit in the UTQG. That'd be AA in our previous example. The traction grade indicates how sticky and capable the tire's tread compound (rubber substance) generates traction.

The conventional approach for traction grading is to slide (not roll) a tire at a steady 40mph over wet pavement. This allows testers to determine how much g-force the tire can produce. The traction grade of a tire is measured when it is slipping instead of rolling. Therefore it doesn't accurately characterize the tread pattern's performance.

It also doesn't include the tire's ability in dry circumstances or during rainy cornering. While limited, it performs an excellent job of measuring the tread compound's quality. On a scale of AA to C, traction grades are assigned. The best traction is usually found on AA tires, while C tires' weakest traction is found.

Temperature Resistance Grade

The thermal resistance grade, denoted by the last character in the series, is the final component of the UTQG. The temperature resistance rating is a straightforward indicator of how well a tire absorbs the heat it creates.

This grade is assigned on a scale of A to C. Tires with an A classification can speed over 115 miles per hour, while rubber with a B rating can travel between 100 and 115 miles per hour. Tires with the least C rating can travel at speeds of 85 to 100 miles per hour.

These speeds represent the efficacy of the tire's thermal dissipation. Poor thermal dissipation restricts the tire's speed potential as more heat is created at more incredible speeds. The Department of Transportation will not approve a tire that does not meet the minimum C rating.

Final Words

As the preceding sections have demonstrated, there is much to learn about what is presented on a sidewall.

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