How To Read Tire Specs (In-Depth Explanation)

If you’ve ever taken a passing glance at the tires on your car, you’ve likely wondered what the numbers and letters littering the sidewall actually mean.

This article will cover everything you need to know about reading and interpreting what is written on a tire’s sidewall.

From aspect ratios to load indexes and even manufacturing dates, we’ve got quite a bit to go over, so let’s get started!

Tire Size Designation

The most important piece of information written on a sidewall is the tire size. Let’s look at how to properly read that first.

In the vast majority of cases, the tire’s size will be expressed as follows: P225/40/R17

Side Note: These numbers will of course vary from tire to tire, but the format will be largely the same.

Let’s break down this size designation in some detail (which is the factory size for a 2012-2015 BMW 328i, in case you’re curious).

The letter “P”

The first important piece of information here is expressed with the letter “P” that precedes the rest of the size designation.

In this instance, the “P” stands for passenger, indicating the tire is intended for use on passenger cars.

Passenger cars would include sedans, coupes, crossovers and smaller SUVs.

Other letters that you may find in place of the “P” include “LT” for light truck, which includes larger SUV’s and pickup trucks.

You may also see a “T” for temporary, indicating a temporary or spare tire being used.

In some cases, you may find tires that have nothing preceding the first number in the tire size designation.

These types of tires are called Euro-Metric, as opposed to their P-Metric counterparts described above.

Euro-Metric tires have slightly different load ratings and inflation pressures than P-Metric tires, but are otherwise considered to be equivalent.

The numbers: 225/40/R17

After the “P” comes the primary tire size information.

The first number of this sequence, in this case 225, is the width of the tire.

The exact width of this first number, is the piece of rubber that actually makes contact with the road.

This number is expressed in millimeters, so the above tire would be 225 millimeters wide.

The next number, in this instance 40, is called the aspect ratio, and this is an expression of how large the sidewall is relative to the width of the tire.

More specifically, a tire’s aspect ratio is the percentage of the width’s size the sidewall is.

In the above example, this tire’s sidewall is exactly 40% as large as the width, which is 225mm.

So, 40% of 225 is 90, making this a 90mm sidewall.

The last number and letter in our size designation provides information about the tire’s construction as well as the diameter of the circle in the center of the tire. This is commonly referred to as the tire’s bead.

We’ll start with that letter R, which indicates that this tire is built using a radial construction, as opposed to a non-radial or bias-ply construction.

Radial tires have separate plies of rubber for the tread and sidewall.

This type of tire is the industry standard on almost all single passenger vehicles.

At times other letters may indicate additional features about the tire, but you’re unlikely to find anything but an R on the majority of tires.

Following the R is the tire’s bead diameter measured in inches, in this case 17. This indicates that the tire will fit on a 17” wheel.

Service Description

The next piece of information on the tire wall is the service description.

This letter and number combo will be denoted by a two- or three-digit number followed by a single letter, 89Y for example.

The service description is used to express two things: the tire’s load index as well as the tires speed rating.

Load index

The numeric portion (89) of the service description in our above example provides details on the load index.

This is the maximum weight an individual tire is capable of safely supporting.

The load index number itself is not representative of the actual weight.

A load index chart must be consulted in order to match this number to a corresponding weight.

In our example, a load index of 89 corresponds to a tire that is capable of supporting 1,279 pounds.

Speed rating

The letter that follows the load index portion of the service description is the tire’s speed rating.

The speed rating, as the name would suggest, is a maximum speed that the tire is safely capable of travelling.

As with load index, the letter doesn’t actually represent a speed, and is instead used to consult a chart which will tell you the actual speed rating.

In the example of 89Y, the tire is capable of supporting speeds over 186mph.

Department of Transportation (DOT) Code

The next significant piece of information that you’ll be able to find on your tire’s sidewall is in the DOT code.

The DOT code is an eight to thirteen-digit code preceded by the words DOT.

It will typically be displayed less prominently than the previous two items we covered, and will be written in a smaller font.

The DOT code is a bit redundant in its expression of four different things: tire size, tire manufacturer, manufacturing location, and manufacturing date.

A typical DOT code will look something like this: DOT 4B08 4DHR 4116.

In the above code, the presence of the letters DOT signifies that the tire they’re printed on meets all DOT tire standards and is compliant with all DOT regulations.

The next two series of numbers, 4B08 4DHR in our above example, tells us the size of the tire, who manufactured the tire, and where the tire was manufactured.

We already know the size and manufacturer provided in the format we already discussed, so this information is a bit redundant.

The one valuable piece of information we can gather is the location of the manufacturing plant.

So if you are trying to find a tire made in the USA, you can look for specific codes on a tire.

The final four numbers in a DOT code, provide us with a tires age, or manufacturing date.

The first two numbers represent which week of the year the tire was manufactured, and the last two numbers represent the year.

So, our example tire whose DOT code ends with the numbers 4116, was manufactured in the 41st week of 2016.

Uniform Tire Quality Grade (UTQG) Standards

The final important piece of information on your tire’s sidewall is known as the tire’s Uniform Tire Quality Grade (UTQG).

The UTQG metric was implemented by the DOT in order to give consumers a more robust understanding of a tire’s capabilities when shopping.

Accordingly, the UTQG describes the tire’s treadwear longevity, as well as traction and temperature resistance capabilities.

A tire’s UTQG rating will be expressed as a three-digit number followed by two to three letters: 300 AA A for example.

Reading Treadwear

The first piece of information that is expressed in a UTQG code is the tire’s treadwear rating.

The treadwear rating measures how long the tire is expected to last, or how quickly the tire will wear out.

While this seems like a useful piece of information on the surface, the way that treadwear rating is measured makes it a nearly useless metric, when comparing tires of different manufacturers.

In order to understand why, let’s take a look at how tire manufacturers come up with their treadwear ratings.

Treadwear Ratings

All treadwear ratings are calculated after a tire manufacture runs their tires through a government test course.

Tire testing begins on a roughly 7,000-mile course to begin gathering data.

After the course is completed, the tire manufacturer will inspect the tire, estimate approximately how long they believe the tire will last, and assign a treadwear rating.

From our example of 300, we can gather the estimate of how long the test tire is expected to last in comparison to the control tire.

So, if our test tire was assigned a treadwear rating of 300, that means that it was estimated to be able to last three times longer than the control tire that it was tested against.

If it were assigned a treadwear rating of 400, that means that it would have been estimated to be able to last four times longer than the test tire, 500 indicates five times longer, etc.

So, if it isn’t immediately obvious, treadwear rating is not a very useful metric for a number of reasons.

Each manufacturer will use a different control tire against which the test tire will be compared.

The result is a treadwear scale varying immensely from manufacturer to manufacturer.

Secondly, the treadwear rating is assigned after an inspection by employees of the manufacturer. Nothing is really preventing them from being as optimistic as they would like in their estimations of tire longevity.

This means that they can grossly inflate the treadwear rating without a significant chance of consequences.

In summary, treadwear rating is a metric spawned of a poorly designed testing procedure, and should be taken with a boulder-sized grain of salt.

Comparing tires from the same manufacturer can be moderately useful.

However, when comparing across brands, it is effectively useless.

Traction grades

The one to two letters that follow the first and only number in the UTQG is the traction grade.

In our above example, this would be AA.

The traction grade measures how sticky and apt at generating traction the tire’s tread compound (rubber material) is.

Sliding (not rolling) a tire at a constant 40mph over a wet surface is the standardized procedure for traction grading. .

Doing so allows testers to measure how much g-force the tire is able to generate.

Because the tire’s traction grade is calculated when it is sliding rather than rolling, the traction grade does little to describe the performance of the actual tread pattern.

It also does not assess the performance of the tire in any dry conditions or during wet cornering.

So, while limited, it does do a fine job of assessing the quality of the tread compound.

Traction grades are measured on a scale ranging from AA down to C.

Typically AA tires having the best traction and C tires have the worst traction.

Temperature Resistance Grade

The final piece of the UTQG is the temperature resistance grade, which is represented by the last letter in the sequence.

The temperature resistance grade is a simple measure of how well the tire dissipates the heat that it generates.

This grade is read on a scale ranging from A to C.

Tires with an A rating are able to travel over 115mph, while tires with a B rating can travel from 100mph to 115mph.

Tires with the lowest C rating can travel between 85mph and 100mph.

These speeds are representative of the effectiveness of the tire’s heat dissipation.

As more heat is generated at higher speeds, poor heat dissipation limits the tire’s speed capabilities.

The DOT will not certify a tire that cannot achieve at least a C rating.

Final Thoughts

As has been amply evidenced by the above discussions, there is quite a lot to know about what is displayed on a sidewall.

Note: Any piece of information expressed on a sidewall that wasn’t included in this article is either redundant of what was included here, or is a piece of completely irrelevant information for a consumer.

Next time you look at the sidewall of a tire you can be better educated on the choice you make.

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