Technical Tuesday - Tire Sizing Explained

Once you’ve fitted aftermarket wheels to your car, you can no longer walk into a tire shop and order tires based on the original manufacturers tire specifications for your vehicle. So, instead of giving the technician at your favorite tire shop a blank look when he asks what size tires you need, read through this article so you can confidently order the perfect tires for your car.

Metric Tire Sizing

A metric tire size is defined in three measurements. The first measurement is simply the width of the tire in millimeters. The second measurement is the height of the sidewall, which is a percentage based on the width of the tire. The third measurement is diameter of wheel the tire can be mounted to shown in inches. These measurements are shown as “Width/Ratio Rim Size”.

An example of these three measurements would be 255/40 R18. In this example, 255 represents the width of the tire, which is 255 millimeters. The second is the tire’s profile, in this case we have a tire profile of 40. The third measurement is stating the tire will fit on an 18 inch wheel.

Tire Type

The beginning of a tire size can vary by region. The letter "P" at the beginning of a tire size means it is a P-Metric Tire. The letters "LT" can appear before or after the tire size and let you know it's a light truck tire. If there are no letters at the beginning of the tire size you know that it is a Euro Metric Tire. 

Tire Width

Tire width is the first measurement you will come across in a tire size. It simply states the width of the tire in millimeters. In our example size of 255/40 R18 we know that the width of the tire is 255mm. 

Aspect Ratio

The second measurement you will see in our example tire size is the aspect ratio. In the example we have a tire size of 255/40 R18. In this measurement 40 is our aspect ratio. The aspect ratio lets you know the sidewall height as a percentage of the tire's width. If you would like to know how this percentage is calculated, how to find the sidewall height, and overall diameter of a tire, read on!

To calculate the sidewall height and ultimately tire diameter, simply multiply the width of the tire by the aspect ratio. In this case we would multiply 255 by 0.40.

(255 x 0.40 = 102)

From this calculation we know that 102mm is the height of the sidewall. To find the overall diameter of the tire from this, multiply 102 by 2 to combine the upper and lower sidewall measurements into one value.

(102 x 2 = 204)

From here we will want to convert the sidewall height from millimeters to inches by dividing 204 by 25.4

(204 / 25.4 = 8.03)

We now know that this tire size will add an additional 8 inches to our wheel’s diameter. In the example of 255/40 R18 we know that the wheel diameter is 18 inches. So, to find the combined wheel and tire diameter we simply calculate the sum of 8 and 18.

(8 + 18 = 26)

From this calculation we now know that the overall diameter of this wheel will be 26 inches.

A few simple measurements of your wheel well will let you know how much tire you can fit under the fender. Utilizing the above calculations you can confidently find the wheel size that is right for your car and order them with confidence.

Tire Construction

This letter will tell you all about the tire's construction. In our example of 255/40 R18, we see that there is an "R" after the aspect ratio and before the diameter. The "R" means the tire is a radial tire.

Wheel Diameter

The final measurement you will see in our example is the Wheel Diameter. This lets you know what size wheel the tire will fit on. In the example 255/40 R18 we see that the wheel diameter for this tire is 18 inches.  

Tools and Resources

Tire and Wheel size calculator 

For anyone that doesn't feel like doing math when looking at tires, there are many online resources that will do these calculations for you based on the width, ratio, and wheel diameter of a tire. 

Check out www.willtheyfit.com to calculate all these dimensions and more with ease. 

Metric to SAE Converter

Check out crawlpedia.com's tire size calculator for a quick and easy way to convert from metric tire sizes to SAE and back again. 


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TechnicalTuesdayDylan Wiebe