How to calculate new tire sizes when changing wheel sizes

Written by justan brandt
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How to calculate new tire sizes when changing wheel sizes
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Whether you’re looking to install a larger or smaller wheel on your vehicle, selecting the right size tire to mount on your wheels is important. Most professionals suggest that your new tire and wheel combination should be within a 3 per cent, positive or negative, variance of the Original Equipment (O.E.).

Skill level:
Easy

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Things you need

  • Original Equipment Tire and Wheel Information

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Instructions

  1. 1

    Locate the first number of your O.E. tire size, this can be found on the side of your tire and should look like 225/45R17.

    225 represents the width across the tire, in millimetres, from the widest point.

  2. 2

    Locate the second number of your O.E. tire size, this can be found on the side of your tire and should look like 225/45R17.

    45 represents the height of the tire’s sidewall, from rim to tread, as a percentage of the tire width. In this case the tire’s height would be 90mm or 3.54 inches, which is 40 per cent of 225 millimetres. This is also known as the tire’s series.

  3. 3

    Locate the third number of your O.E. tire size, this can be found on the side of your tire and should look like 225/45R17

    17 represents the wheel diameter, in inches, that the tire is intended to be mounted on.

  1. 1

    Calculate the overall tire height, in inches, using the following formula ((Width/25.4)Sidewall Percentage)2

    This means a 225/45R17 tire would have an overall tire height of 7.09 inches, ((225/25.4)40%)2).

  2. 2

    Combine the overall tire height with the wheel diameter to reach the final diameter of the wheel and tire combination.

    Using the previous example of, 225/45R17, you would add 7.09 to 17 for a final diameter of 24.09 inches.

  3. 3

    Calculate your acceptable tire and wheel diameter using the generally accepted rule of a 3 per cent variance.

    Using the examples from above you would be able to install a new wheel and tire combination that was anywhere from 23.37 inches in total diameter to 24.81 inches in total diameter.

Tips and warnings

  • Ensure the width and offset of your new wheels will fit your vehicle properly as well.

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