How to Read Tire Size: Decoding the Numbers and Digits on Your Sidewall

Fitting the correct size tires to your car is crucial. Get this part wrong, and you’ll have a whole lot of problems coming your way. It isn’t just important for the tires to fit, but it’s also vital for you and your passenger’s safety, as well as the performance of your vehicle.

When you’re buying new tires, however, there’s so much more to consider. You have to take into account speed rating, load index, aspect ratio, wheel diameter, and the conditions you’ll be driving in. 

Sound confusing? Don’t worry – we’ve got your back! In this article, you’ll find out how to read tire size, where to find this information, and how to decode all the other numbers and digits on your tires’ sidewalls. 

Where to Find the Tire Size

No matter the make and model of your vehicle, the owner’s manual will contain information about the tire size your car needs.

If you’ve misplaced your owner’s manual, there are other places to find this information. These spots include the inside of the glove box door, inside the gas tank hatch, or even the driver’s side door jamb.

In order to determine the size of the tires currently present on your vehicle, it’s embossed on the tire sidewall markings, along with other important information.

How to Read Tire Sizes

At first glance, the combination of numbers and letters that you come across might feel like you’re learning a new foreign language. That’s really not the case, though. With the right information and once you know what you’re looking at, it’s actually quite easy to understand.

Let’s go through this information in order, using the following example: 

This is broken down into the following areas:

1. Tire type.
2. Tire’s width.
3. Aspect ratio.
4. Construction type.
5. Wheel diameter.
6. Service description
   1. Load index.
   2. Speed rating.

The First Letter: Tire Types — P225/50R17 98H

It’s very common for the tire markings to start with a letter. This letter varies and indicates the type of vehicle that the tires are made for.

In our example, the P stands for P-Metric. This signifies that the tires are made for a passenger vehicle.

The letters and their meanings are:

• P: Passenger tires.
• LT: Light truck
• T: Stands for temporary and is usually on spare tires.
• ST: Tires intended for a special trailer.
• No letter: Eurometric tires

Load Ranges

The letter indicating the type of tire may have some additional markings that show its load range. This is the amount of load that the tire is able to carry with the industry-specified pressure, which is standardized by the Tire and Rim Association.

If you have passenger car tires (P), then they have named load ranges. Meanwhile, the light truck (LT) tires have alphabetical load ranges.

So, if you’re planning on changing tire size, you should check that the load index is either equal to or bigger than the one of the original tire. You should also check that the new tire has a maximum load capacity that will be enough to carry the entire car’s gross axle weight rating.

Passenger (P) Tire Load Indicator

Most of the time, you’ll see the P on its own.

However, sometimes, the tires can be branded to show extra load, with ‘XL’ in the descriptions (or ‘RF’ for reinforced load for any European tires), or a light load with ‘LL’ in the description.

Light Truck (LT) Tire Load Indicator

Light truck tires have various load ranges. This is shown right after the number of the size of the diameter in the description. These tires can be branded with either their load range letter or their ply rating, and you can see that in the table below:

The Next Few Numbers: Width of Tire—P225/50R17 98H

The numbers indicating the width of the tire are found after those that indicate the tire type (if there is one), and before the forward slash symbol.

This number is the tire width, in millimeters. The measurement is taken from one sidewall to another.

Numbers After the Slash: Aspect Ratio—P225/50R17 98H

After the slash, there is another set of numbers. These numbers are for the tire’s aspect ratio and are in percentages. The aspect ratio shows you the height of a tire’s sidewall in relation to its width.

Tire makers calculate this by dividing the tire’s height by its width. In our example, the aspect ratio is 50%, meaning that the sidewall height is half the size of the width.

The Following Letter: Internal Construction Type/Tire Type—P225/50R17 98H

After the aspect ratio, you’ll find a single letter, which indicates the construction material of the tires.

The two letters you are likely to see are:

• R
• D

Construction Type R (Radial Construction)

This is a radial tire and has additional layers of fabric. The fabric threads, which run all the way around the tire, provide additional strength to radial tires.

Construction Type D (Ply Cords)

If you see the letter ‘D’, it means that the tire is built using ply cords. The tire’s internal ply cords are extended diagonally, from bead to bead.

The Next Two Digits: Diameter—P225/50R17 98H

After the tire’s construction type, the next two numbers signify the wheel diameter that the tire was intended to fit.

This length is specified in inches and is the rim diameter of the intended wheel, onto which the tires are mounted.

In our example, the tire was intended to fit a wheel that is 17 inches in diameter.

Service Description—P225/50R17 98H

Following on from the diameter number, there will be two numbers followed by a letter. This is known as the service description of the tire and is broken down into load index and speed rating.

Load Index—P225/50R17 98H

The digits after the diameter number are the tire’s load index.  The load index number corresponds to how much weight a tire is able to support when inflated to maximum capacity.

The load index measurement is in pounds, so our example indicates that the tire has a load index of 98. Using our table below, we are able to ascertain that the tire is designed to support 1,653 pounds at maximum inflation.

The numbers for the load index start from 1 and can go up to 150 in the index, and they represent weight-carrying capacities from 99 up to 7,385 pounds. However, the usual load index that you will find on passenger cars’ and light-duty pickup trucks’ tires ranges between 70 and 126.

Additionally, if you have light truck or special trailer tires (LT/ST), there are two separate numbers for the load index on the sidewall of the tires.

The first number here shows the weight-carrying capacity if the tires are on a truck that has a single-wheel rear axle. Meanwhile, the other number is for when you’re using the tire with a dual rear application.

Usually, with these markings, the tires are rated so that they carry a lot less weight when they’re working together with another tire, in a dual pair. This is so that if one of the tires ends up failing, the other tires can carry the rest of the load normally. It’s a way for manufacturers to build in some reserve capacity for the tires.

Here’s the load index table for passenger cars and light trucks:

Speed Rating—P225/50R17 98H

Following the load index, you’ll find the speed rating letter. This is the maximum speed capability of the tires, which is derived from rigorous testing.

However, you should remember that this isn’t a recommendation for how fast to drive with these tires on your vehicle. You should always be following the legal speed limits and not drive up to the maximum vehicle speed capability, no matter where you are.

The speed rating was established so that tires can match the speed abilities of the vehicles they are used with. The number is determined by running tests on the tires, where a load is placed on them and it is then tested at increasing speeds.

However, this rating only applies to the tires if they haven’t been damaged or altered in any way.

In our example, the ‘H’ indicates a speed rating of up to 130 mph. Remember, this is a safe speed limit for the tire, and it is NOT an indicator of how fast you can or should drive.

Here is the table to translate the letters behind the speed ratings:

Finally, there’s also the letter ‘Z’, which is rated for 149+ mph. This will often be found in the wheel size area. W or Y will still be included in the speed rating area of the service description.

Other Markings and Information

Now that we’ve covered the letters and numbers that indicate all there is to know about tire size, load ratings, speeds, etc., it’s time to look at the other information you might find embossed on tires:

• DOT number.
• UTQG.
• Maximum inflation.

DOT Identifier

All of the tires in the US must carry a DOT number. This is a Department of Transportation number, and it shows that the tire in front of you has passed all of the minimum standards for sale set by the Department of Transportation.

It’s branded with the letters ‘DOT’, followed by the various codes set by the department.

This is a total of 13 letters and numbers that show the location where the tire was made, tire size, and the code of the manufacturer, as well as the year and the week it was actually made.

So, you can easily find when your tire was made by looking at the last four digits. The first two numbers identify the week of the year, and the last two identify the year.

For example, if you see 48/09, it was made during week 48 of the year 2009. And if your tires were made before the year 2000, there will only be three numbers, with the last one identifying the year.

Uniform Tire Quality Grading Marking (UTQG)

Then, you may also find the Uniform Tire Quality Grading Marking (UTQG).

This is another code that the Department of Transportation requires to be placed on tires. It is a system that has a rating for traction, treadwear, and resistance to temperature.

• Traction Marking: The traction rating shows the ability of the tire to halt in a straight line on wet roads or concrete. The grades range between AA, A, B, and C.
• Tire Treadwear Rating: Treadwear rating shows the durability of your tire, but do note that the tire has experienced minimal treadwear in controlled testing conditions.
• Temperature Resistance: The temperature rating shows the resistance of the tire to the generated heat when you’re using it at high speed. These ratings range between A, B and C.

Maximum Inflation Pressure Number

This number shows the maximum air pressure that the tire is able to hold while it’s being used. This is measured as PSI (pounds per square inch).

It’s important to note that this isn’t an indicator of what to inflate the tire pressure to for your specific vehicle. Consult the car manufacturer’s recommendation for this level.

If your car manufacturer indicates a PSI of 32 and your tire has the number 30 embossed on it, you should not be inflating the tire above 32 PSI. Seek an alternative tire that suits your needs.

Now You Can Read Any Tire, Anywhere!

Made it this far? Good job!

You now know all about how to read tire sizes, and you should find it much easier to ensure that you have the correct tires for your vehicle’s needs.

Alternatively, you can always just go ahead and show off your knowledge to your friends and teach them everything you’ve learned today, just in case you know someone else who’s confused about all of this as well.

Either way, now that you know all there is to know about the branding on the sidewall of every tire, you can crack any code, and you won’t even need your owner’s manual to check all the codes anymore.

Go out there and get code cracking!