You can describe tire sizes for vehicles such as cars, trucks and SUVs using several different standards. Commonly used standards include standard inch size and metric size. Standard inch size may look like 31×10.50r15 while metric sizes will be closer to 265x75r16, although these numbers will vary depending on the size of your wheels and the bolt pattern. There’s also a third standard called Military/Farm size, which as you may have guessed describes the tire sizes of tractor tires and other military equipment.
Tire Sizes
Standard inch size can be broken down into three separate components that describe the actual size of the wheel. For example let’s take a look at 31×10.50r15. The first number, 31, represents the overall diameter of the tire. The second number, 10.50, tells us how wide the tread is on the tire. The last component, r15, describes the size of the rim the tire is made for, in this case r15 represents a 15 inch rim.
Metric tire sizes are a little less straight forward than stand inch sizes. They are actually the combination of inch sizes and metric sizes. Let’s take a look at 265x75r16 for example. Metric sizes are measured in what is known as the modified metric system or US metric system. In this example the 265 tells us that the width of the tire tread is 265 millimeters. The next number, 75, describes the height of the tire’s sidewall as a percentage of the width of the tire. In our example, the tire sidewall is 75% of 265mm, or 199mm. The third number is identical to the stand inch size and represents the size of the wheels that the tire is made for; in this case r16 means the tire will fit on 16 inch rims.
Metric to Inch Tire Size Conversion
The most important thing to remember when converting tire sizes from metric to inch and vice versa is that an inch is equal to 25.4mm. Let’s use 265x75r16 again as an example. 265 divided by 25.4 is equal to 10.43 inches.
Finding the height of the tire is a little trickier. You must take the width (10.43) and multiply it by the sidewall percentage (75%). 10.43 times 0.75 is equal to 7.82 inches. You must then add this number to the size of the rims (r16 is 16 inch rims) to get the overall diameter of the tire. In this case it would be 7.82 inches + 7.82 inches (top and bottom sidewall) + 16 inches equaling 31.64 inches.
Based on these calculations, 265x75r16 in metric is equivalent to 31×10.50r16 in standard inch size.
Inch to Metric Tire Size Conversion
This is basically the opposite of what we did above, although I’ll walk you through it to clarify the process. Let’s use 31×10.50r16 again as an example. Calculating the width of the tire is easy, just multiply 10.5 inches by 25.4mm (number of millimeters in an inch) to get 266.7mm. Metric tire sizes will typically be rounded to equal divisions of 5, giving us 265mm in this case.
To determine the sidewall height we simply take 31 inches (overall diameter) subtract 16 inches (rim size) and divide by 2 to give us the sidewall height. In this case that would be 31 – 16 (15) divided by 2 equaling 7.5 inches. We can then find the percentage value by dividing the sidewall height by the tread width. 7.5 inches divided by 10.5 inches equals about 71%. Percentages are also rounded up in metric sizing, giving us 75%. This makes our metric tire size 265x75r16.
Tire Size Conversion Chart
You can use a tire size conversion chart to eliminate the complicated math involved with tire size conversion. A chart will easily lay out all the common tire sizes allowing you to quickly see the converted size whether you are going from inches to metric or metric to inches.