How to Change a Car's Tire Size
One of the most common modifications on an automobile is to swap out small factory rims with larger rims. However, to avoid losing power and affecting your speedometer, the tire size must also be changed. For example, let's look at the common tire size 215/65 R16. A common upgrade would be to the tire size of 255/35 R20 because they are the same overall height of 27 inches. Since rims are measured in inches and millimeters, some math must be done to determine the tire size change that will work for you.
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Instructions
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Multiply the 20-inch rim size by 25.4 to get your overall rim size in millimeters. According to OO Kingdom, 1 inch equals 25.4 millimeters. In keeping with our example of going from a 16-inch tire to a 20-inch tire, the tire size of 255/35 R20 will be used throughout. Multiplying 20 inches by 25.4 millimeters gives you 508 millimeters. This is your overall rim size in millimeters.
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Determine the size of your tire's wall in millimeters. In our example, the width of the tire is 255 millimeters. A 255/35 tire means the wall of the tire (area from outer edge of tire to inner edge of tire rim) is 35 percent (or 0.35) of 255 millimeters. If you multiply the tire width of 255 millimeters by its aspect ratio of 0.35 you'll get 89.25 millimeters for a side wall. Since a tire has a top and bottom side wall you need to multiply the size wall by 2. In our example you'll get a total of 178.5 millimeters for both size walls.
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Add the overall wheel height and overall wall height to obtain the overall tire height of your new wheel and tire combination. For example, 508 millimeters added to 178.5 millimeters equals 685.90 millimeters. Now convert your total millimeters back to inches. For example, 686.5 millimeters divided by 25.4 millimeters equals 27.00, meaning the current overall size is 27 inches. You have changed your tire size from 215/65/ R16 to 255/35 R20 which means you now have a 10-inch wide, 27-inch tall tire.
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Tips & Warnings
Some car owners like to go with a different overall tire height. This can affect the gear ratio in your rear end which can affect low-end torque or high-end horse power.
There are a lot of online calculators such as 1010 Tires that can help you if you are going with a standard size change, but for the odd changes you'll want to be able to do the math.
