How To Read Tire Size

Reading the Tire Sidewall

Reading the tire size can be confusing. There are some numbers and other letters mixed to form a string of characters that means nothing to the untrained eye. Fortunately, we can help.

The tire size reads from left to right on the tire and most commonly is 12 characters long. This includes many different sizes and spec indications about the tire too. There are seven different tire size meanings that we will discuss below.

Tire/Vehicle Type

The first letter found at the start of the tire sidewall is the tire type. It can also indicate the type of vehicle you are driving.

The tire type indicates the type of vehicle this tire would fit best. Two different letter combinations would appear here for you to understand.

The letter P stands for passenger vehicle tire. A passenger vehicle is a traditional option like a car, SUV, crossover, minivan, and some smaller pickup trucks. Passenger vehicles dominate the market and are found on a majority of tires.

The letters LT stand for a light truck tire. A light truck vehicle is exactly what it sounds like, but it is defined as a vehicle that is towing a trailer or maintains a larger load capacity. The load capacity baseline for an LT vehicle is between 3/4 to 1-ton of load capacity.

Another combination that could appear is ST which stands for a special trailer. This is for tires that specifically fit onto trailers for any type of usage.

European-sized tires do not include a tire type at the start of the tire size. This is known as a metric tire.      

Tire Width

The tire width is the number that directly follows the tire width letter. It is a three-digit number that measures the width of the tire in millimeters.

The tire’s section width is measured from the outer sidewall of the tire to the inner sidewall. This is a complete measurement that gives us the exact width of the tire.  

Aspect Ratio

The aspect ratio is shown as a two-digit number following the forward slash. The number is displayed as a percentage to represent the profile of the tire.

The aspect ratio is the height of the sidewall measured from the wheel rim directly to the top of the tire tread. Because it is a percentage, the equation is shown as sidewall height divided by tire width.

Construction

The construction type is an essential factor that tells you about the internal construction of the tire. There are two different types of tire construction: Radial (R) and Bias (D).

Radial Tire

Radial construction is for radial tires and represents the tire industry standard in the modern era of vehicles. These tires provide more traction on the road with reduced rolling resistance. They are also more comfortable with better durability.

The radial tire construction consists of a cap play, steel belts, radial plies, and a rubber-coated fabric. This entire construction mold lays horizontal on the tire to blend into the treads seamlessly.

Bias Tire

For a bias construction tire, you would see the letter D on your tire to represent the diagonal construction of the plies. Unlike radial, the bias plies lay crisscrossed inside the tire. It is a more conventional design that some motorcycle and trailer tires still use.

Rim Diameter

The rim diameter is also referred to as the wheel diameter. This number is measured in inches as a two-digit number directly after the construction type.

To measure the wheel diameter, it measures horizontally across the tire between the bead seat areas. This is the location of the tire where it gets sealed onto the wheel. There is a misconception that it measures all the way to the edge of the wheel, but this is incorrect.  

Load Index

The load index is a two or three-digit number that follows the rim diameter. The load index represents the amount of weight a tire can support. The number of the load index is a symbolic number that pairs with a maximum load in pounds.

The chart below shows every load index that can be displayed on a tire. However, only a handful of tire load indexes are shown on traditionally manufactured tires for vehicles.

Speed Rating

The speed rating is the final aspect of a tire size. It is a letter shown after the load index to indicate how fast a tire can drive. It displays the top speed.

The higher the speed rating on a tire, the more heat the rubber on the tire can handle. When tires drive fast, they cause heat to build up, and if the rubber isn’t specially made to handle that heat, the tire will fail.

Following your tire’s speed rating is very important; otherwise, you could potentially damage your tires or your vehicle. The chart below displays all speed ratings that exist, while only five or six appear on vehicle tires regularly.  

What Does it All Mean?

That may seem like a lot of information to handle. So many numbers and letters being thrown at you that you may have been confused. Let’s look at an easy example to understand fully.

Assume you see a tire with the following size: P215/65R1890H

We start by identifying that this is a passenger vehicle tire type.

The tire width is 215 millimeters. Now we move to the aspect ratio, which is 65. This means that the tire sidewall height is 65% of its width or 139.75 millimeters (5.5 inches).

Next, we see the letter R indicating this is a radial tire.

The wheel diameter is the two-digit number 18, meaning it is 18 inches wide.

The load index on this tire is 90 meaning the maximum load capacity is 1323 lbs.

The speed rating is H meaning the top speed on this tire is 130 MPH.

The best way to start is to take each step one by one. Tying it in together becomes easy after a few tries. We hope this helps with your next tire purchase!