How to Downsize Tire Sizing for Winter Tires

by Justan Brandt
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It is common practice for vehicles equipped with performance, or all-season, tires to switch to a set of winter tires for the cold and snowy seasons. All-season tires provide vehicles with maximum wear through any weather conditions. High-performance tires provide maximum traction under high-temperature conditions. These types of tires are not designed to provide you with maximum grip in snowy, cold or icy conditions; snow tires are. To select a set of snow tires and wheels, you need to calculate the overall diameter of the original equipment tires and stay within a 3 percent variance.

Reading Tire Size's

Step 1

Locate the O.E. tire size. This is found on the sidewall of the tire and will look similar to: 255/50R17.

Step 2

Note the width of the tire from the first number in the tire size. In this case, 255 represents the width across the tire, in millimeters, from the widest point.

Step 3

Calculate the tire sidewall height. In the example tire size, 255/50R17, 50 represents the height of the tire’s sidewall, from rim to tread, as a percentage of the the tire width. In this case the tire’s height would be 127.5 millimeters (50 percent of 255), or 5.02 inches. This is also known as the tire’s series.

Step 4

Locate the O.E. wheel diameter; this is the last number in the tire size. In the example, 255/50R17, 17 represents the wheel diameter, in inches, on which the tire is intended to be mounted.

Calculating Tire Diameter

Step 1

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

A 255/50R17 tire would have an overall tire height of 10.04 inches: ((255/25.4) x 50%) x2.

Step 2

Combine the overall tire height with the wheel diameter.

Using the example of, 255/50R17, you would add 10.04 to 17 for a final diameter of 27.04 inches.

Step 3

Using the 3 percent variance rule, you would be able to install a new tire and wheel combination that was between 26.23 inches and 27.85 inches in total diameter.

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