How to Calculate the Approximate Overall Diameter of a Tire

There may come a time when you need to know the actual overall diameter of a tire. There is nothing on a tire that will give you this information. You simply have to know the formula and the tire size. Tire sizes are in millimeters, and the rim, or wheel, diameter is in inches. For example, on a 235/45/17 tire, the first number is the width of the tire in millimeters. The 45 represents the sidewall (as a percentage). On this tire the sidewall is 45% of 235 millimeters, (105.75 millimeters). The final number is the wheel size. In this case it is a 17 inch wheel.

Difficulty: Easy

Instructions

Things You'll Need:

• Tire size
• Calculator

1. First determine the width of the tire in inches by taking the width in millimeters and dividing by 25.4 to convert to inches. Let's use the same tire in the example above. (235/45/17) The math would be 235 / 25.4 = 9.25. This means that the tire is 9.25 inches wide.

2. Multiply the width in inches by the aspect ratio, or sidewall size as a decimal (.45). Then multiply that number by two. 9.25 X .45 X 2 = 8.33. You multiply by two because there are two sections of sidewall from the ground to the top of the tire. This number is the height of the sidewall of the tire. There are 4.165 inches of tire on each side of the rim.

3. Finally, add your answer from step 2 (8.33) to the rim diameter (17 inches). 8.33 + 17 = 25.33. Now you have your approximate overall tire diameter. As you can see, the tire is actually quite tall even though it is a 17 inch rim. This will help you figure out how much you can "inch up" (or how much of a bigger wheel you can have) when you change your wheels and tires. You can measure the height of the wheel well by measuring from the middle of the wheel well to the ground. This measurement will help you determine what size tires and wheels your vehicle can accommodate. Remember to take into consideration that this measurement is while the vehicle is at rest. When you go over bumps or add weight to the vehicle the gap between the wheel well and tire will become much less.

Tips & Warnings

• This is not an exact measurement. It is only approximate. However, it is close enough to help you find the perfect wheel and tire size for your vehicle.

[Upon-Request]

on 9/5/2009 Thanks for the info!