How to Find Tire Diameter

How to Find Tire Diameter

If you want to get a new spare tire cover for your car, the first thing you need to know is how to find the tire diameter. 

Many tire specifications won’t make that much of a difference when it comes to compatibility, but you can’t fit a tire with a smaller or larger diameter than what your cover is supposed to take. 

Luckily, you can learn what size a tire’s diameter is by taking a quick look at the tire code engraved on the sidewall and doing some simple calculations. So, let’s find out how!

What’s the Tire Diameter?

Tire diameter is the length of any straight line that passes through the center point of the tire and whose endpoints are on the outer edge of the tire. You can measure tire diameter in inches or millimeters. 

how to determine tire diameter

The Tire Code

Before calculating a tire’s diameter, you need to be familiar with the standard tire code first. It’s the code that helps you tell the size of the tire and other essential specifications. You can locate the tire code on the sidewall of any modern tire. 

An example of a standard tire code is “P 185 / 75 R15 82 S”. Each number or symbol in this code indicates a specific tire measurement or specification. Now, let’s break it down into separate sections to understand what each number or letter means:

Tire Type

The tire type is designated by one of the following subcodes:

  • P for Passenger 
  • LT for Light Truck
  • T for Temporary 
  • ST for Special Trailer

In our example, the code includes the letter “P”, which means the tire is made for passenger vehicles only, like sedans and coupés. 

 how to measure tire diameter

Tread Width

The first number on a tire code indicates the tread width at the thickest point in millimeters. In our example, the tread width is 185mm, or 7.283 inches. You can easily convert tire tread size to inches by dividing by 25.4. 

Aspect Ratio

The aspect ratio is the ratio between the tire width and height. You can use it to find the height of the sidewall. In our case, the aspect ratio is equal to 75%. 

Internal Construction 

The second letter in a tire code indicates the internal construction of the tire. “R” means that the tire is of the radial type, while B is for Bias-Belt. A third, less-common type is “–”, which means that the tire is a Bias-Ply one. 

In our example, the letter is “R”, so the tire’s internal construction is Radial. 

Rim Diameter

The rim diameter is the diameter of the wheel that the tire is designed to fit on in inches. In this example, the rim diameter is 15 inches.

However, keep in mind that rim size doesn’t equal the overall diameter of the tire. It’s the size of the rim that can take this tire.

Load Index

what is the diameter of a tire

The load index indicates the maximum amount of load a tire can withstand. In this example, the load rating is 82, translating to a weight-bearing capacity of 1087 lbs per tire.

You can then get the total load rating of the whole tire set by multiplying that number by 4.

Speed Index

The speed index is how fast you can go without affecting the tire’s heat dissipation efficiency. It’s denoted by one letter from “L” to “Z”. In our example, the “S” symbol means that the tire’s recommended maximum speed is 112 mph.

How to Determine Tire Diameter

Now that you can read the tire code, you can easily calculate the tire diameter with this simple formula:

(Sidewall Height x 2) + Rim Diameter = Total Diameter

Remember, you can calculate the sidewall height as follows:

Aspect Ratio x Tread Width = Sidewall Height

In our example, the sidewall height is calculated as follows:

0.75 x 7.283 = 5.462 inches

Now that we have the sidewall height, you can go back to the first formula and calculate the total diameter like this:

(5.462 x 2) + 15 = 25.92 inches

If you don’t want to do the manual calculations yourself, you can use an online tire diameter calculator

How to Measure Tire Diameter – An Alternative Way

Sometimes, if a tire is a bit old, you may not be able to see the tire code clearly due to wear. In that case, you can simply use a tape measure to find out the diameter of the tire.

Conclusion

Calculating the tire diameter isn’t that complicated. After you figure out how to read the tire code, the calculations won’t take a minute. 

Once you know the diameter of your spare tire, getting a new spare tire cover should be pretty easy for you!

Did you find this guide helpful? Then consider checking others:

How To Measure Tire Rim Size
How To Convert Tire Size To Inches
How Do You Measure Tire Size
How To Find Out Tire Size
How To Read Trailer Tire Sizes