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If anyone is looking for a good tool for calculating tire sizes, you can use the site https://www.willtheyfit.com/ that gives you all the info you need.

If you're looking to do the math yourself for figuring out offsets, here's what you need to know.

The offset calculation is taken from then centerline of your wheel to its mounting hub face.
  • 0 offset would be the centerline is inline with the hub face.
  • Positive offset means the hub face is closer to the outer edge of the rim.
  • Negative offset means the hub face is closer to the inner edge of the rim.
Here's an example:

OEM Si wheels they're 8" wide with and offset of +50.

If you're switching to a set of wheels that are 81x8.5 +30 the new wheels are 0.5" or 12.7mm wider than OEM. If the new wheel offset was +50 the outer edge of the rim to the centerline would be 6.35mm closer to the fender. Since the new wheels are +30 (more negative) the new wheel is 26.35mm closer to the fender.
 
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