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A real accurate way to determine your tire diameter is to draw a line on the bottom of your tire, then draw a line on the ground which lines up with the line you just drew on your tire.
Then move the car until the car makes one full rotation of the tire and mark the ground where the line on the tire is at the bottom again.
Now measure the length between the two lines and divide by pi (3.14) and you'll have your tire diameter to use on the calculator (don't forget to use inches).
This is the preffered method to use in determining your actual tire diameter.