How to Convert Winter Tires to a Smaller Size

Snow tires provide additional traction in winter conditions.

When installing winter tires on the rims of a vehicle, always use the exact size recommended by the manufacturer. When special cases arise, such as unusual sizes or installation of smaller rims, you may need to consider an alternate tire. A general rule of thumb is to keep the overall diameter of the rim and tire within a 3 percent variance of the original equipment.

Instructions

1. Locate the first set of numbers of the OE tire size. For example, with the 255/50R18 tire size, the first set of numbers would be the "250." This is the widest section of the tire in millimeters. Dividing the width by 25.4 will convert the measurement from millimeters to inches; 250 mm divided by 25.4 equals 9.84 inches.

2. Notate the number following the width of the tire. In the example, 250/50R18, this would be "50." This is the height of the tire's sidewall, represented as a percentage of the tire's width. Using the example, the height of the sidewall is 50 percent of 250 mm, or 125 mm. This could also be represented in inches by taking 50 percent of 9.84 inches.

3. Read the last numbers found within the tire size. For a 250/50R18 tire, the last available numbers are "18." This represents the diameter, in inches, of the rim the tire is designed to be mounted on.

4. Calculate the overall diameter of the tire and rim. This is accomplished by adding the sidewall height to the rim's diameter. The example tire size has a total sidewall height of 9.84 inches, or 4.92 inches multiplied by 2. Multiply the sidewall height by two to compensate for sidewall all the way around the tire. The height of the sidewall can then be added to the rims diameter. In the case of our example, 18 plus 9.48 equals a final diameter of 27.84 inches.

5. Calculate the allowable variance. Using the 3 percent rule of thumb, a new tire and rim combination with a diameter of 27 to 28.68 inches would be appropriate for our example tire.

Tips & Warnings

• Before installing a new rim or tire, consult with the service department of your vehicle's local dealership.

References

• Tire Rack: Tire Size Guide
• Tire Rack: Calculating Approximate Tire Dimensions