How Are Tires Measured?

‍How Are Tires Measured?

As a vehicle owner, understanding tires and how they are measured is important to know better how your car works. It also makes shopping for new tire upgrades easier.

Tires are measured in inches and millimeters depending on the variable. And when you see the measurements, they could be referring to a few different factors that make up a tire size.

Both the width and the height of the tire are measured in millimeters. The rim size is measured in inches to make it easier for drivers to pair tire sizes with their vehicles in the United States.

Tire Width

The tire width is referred to by a three-digit number indicating the tire size from one side to the other. This is measured in millimeters and spans from one sidewall to the other.

This is arguably the most critical number to remember because it is the easiest way to refer to tire size and the first number on the side of the tire.

For example, a tire that reads P225 would refer to a passenger tire that is 225 millimeters wide. This is a common size for many vehicle types.

Tire width is one of the biggest tire size factors regarding price changes, road grip, road noise, and overall driving comfort. It also impacts the appearance of the tire too.

Rim Size

The rim size is measured in inches, which is important because it determines the tire's width. The larger the rim size, the wider the tire will be.

And this also typically means it will fit on a bigger vehicle. The rim size impacts a tire's performance because it determines how much air can be put into it.

Different rim sizes are used for different purposes. For example, if you want to drive on rough terrain, you will need a wider rim size than an average car.

Tire Height

The tire's height is usually measured from the ground up to the top of it when it's sitting on its rim, including any part of the rim that sticks out past the edge of the tire.

However, the tire height is unique because of how it is expressed by using a ratio. This aspect ratio gives a percentage of the tire width to indicate the exact height of the tire.

Tire height is so important because, without the proper ratio, you would struggle with balance and stability while driving. For example, a higher aspect ratio usually means a wider turning range.

How Do You Read The Tire Size?

The tire size is a string of characters on the side of a tire, and each has a specific meaning. Understanding it would be nearly impossible if you have never seen this before.

There are seven different characters on the side of the tire. We have listed each below, so you will fully understand what those characters represent when you look at your tire sidewall next time.

Vehicle/Tire Type

You will first see a letter indicating the vehicle and tire type on your tire. A few different potential letter combinations could appear depending on the type and size of the vehicle you drive.

This includes P, LT, and ST, but the most common tire is a P. This passenger vehicle tire works for sedans, SUVs, compact pickup trucks, and more.

An LT tire is a light truck tire that can hold heavier weight capacities on bigger trucks. They are also commonly used on towing trailers.

The ST tire is for special trailers. This is less common, and they are not used on vehicles.

It is also possible to have a tire without a letter, but it is unlikely in the United States. This means you have a European metric tire.

Width

Next, you will see a three-digit number representing the tire's width. This measurement indicates the total width in millimeters, so a P225 size would mean a passenger tire that is 225 milliliters wide.

This measurement is taken from one endpoint of the tire to the other. It is also referred to as the section width of the tire.

Aspect Ratio

A tire's aspect ratio is a measure of its height relative to its width. The number is two digits because it indicates the percentage relation.

For example, a tire that reads P225/60 means the aspect ratio is 60. This means you would need to multiply 225 by 60% to get the total height of the tire.

Construction

The next character you will see on the tire sidewall is less about the size of the tire and more about how the tire is made. This is known as the construction type.

The two common options are radial tires and bias tires. You will see the letter R on a radial tire and the letter D on a bias tire.

Rim Diameter

Another important factor in tire size is the rim diameter. When matching tires with different vehicles, the rim diameter is always a two-digit number.

This is the measurement from the endpoint of the tire's rim. It is indicated in inches on the tire sidewall.

Load Index

The load index is an important factor to consider when purchasing a tire. It tells you how much weight the tire can carry, which will help you determine if it’s right for your vehicle and driving needs.

It is also displayed as a two-digit number. You can use this chart to convert this into a weight in pounds.

Speed Rating

Lastly, you will see a single letter at the end of the tire size. This indicates the speed rating of the tire and the maximum speed it can handle.

This speed rating chart will convert this into MPH, so you know exactly what type of performance to expect from the tire.

How To Measure Tires Yourself

If you were in a situation where the tire sidewall no longer indicated the size, you could do this by yourself too. It is quick, easy, and can be done in a few short steps.

First, you would need the proper equipment to handle the tire, like a tape measure and a flat surface. You can begin by laying the tire flat with the open rim area facing upwards.

Start by measuring the tire from top to bottom to get an accurate idea of the tire’s height. Be sure to write down the measurements as you go.

Next, you can continue by measuring the wheel diameter by measuring the width of the tire hubcap. Try to take this measurement at the proper angle without bending the tape measure too.

Now, you can stand the tire up and measure the width of the treads too. This is done by measuring from one endpoint to the other.

After getting these three measurements, you can make the proper conversions and calculations to determine the tire size.