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Metric Tire Conversion:
Most of us running 15" wheels don't have to worry about metric tire sizes, but for those with larger wheels man times only metric tire sizes are available. FAQ: Understanding Tire Sizes will help explain metric tire sizes.
I have an Excel Spreadsheet that converts RPM to MPH based on tire size and gear ratios, that includes a standard to metric tire converter, which can be downloaded here. (If you don't have Excel, it also works great in OpenOffice which is free to download.)
Now we'll walk through on example, just so you understand what's going on in the conversion process. Lets take 265/70/16 as an example.
1. The first step is to convert the 265 (width of the tire in millimeters) to inches. Divide the 265 by 25.4 which gives us 10.433
2. Next 70 is a percentage of the width. Take the 10.433 x 70%, which would be 10.433 x 0.70 which gives us 7.303 This is the sidewall height.
3. Next take the sidewall height, and multiply it by 2. (Remember you have the sidewall, wheel, and then the side wall on top) So you have 7.303 x 2 which gives you 14.606.
4. Finally take the 14.606 (height of the top and bottom sidewalls) and add them to the height of the wheel, in this case 16 (the last number in the metric tire size). This gives us 30.606
Result: A 265/70/16 is a 30.606 x 10.433 x 16, or pretty close to a 31 x 10.50 x 16
Here is a chart of common sizes that maybe handy.
30x 9.5x16 - 245/75/16
31x10.5x16 - 265/70/16
33x12.5x16 - 305/70/16
35x12.5x16 - 315/75/16
31x10.5x17 - 265/70/17
33x12.5x17 - 305/65/17
35x12.5x17 - 315/70/17

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