[Wavery]

Originally Posted by wmtire

I don't have a measurement, but there is a formula that we use to get a real close approximation.

Say for example you have a metric sized tire of 225/75R15. The first number (225) is the section width. The second number (75) is the aspect ratio and the last number (15) is the rim diameter.

Here is the formula

(Section Width X Aspect ratio X 2) divided by 2540, then add rim diameter= approx tire diameter

(225 X 75 X 2) =33750

33750 divided by 2540 = 13.287

13.287 + 15 = 28.287 inches is approximate tire diameter.

You multiply the section width and aspect ratio by two, to give you the height of the two (or top and bottom) sidewalls of the tire.

You divide this result by 2540 because the tire size section width to begin with was in millimeters (225 millimeters). There are 25.40 mm to an inch, so we are converting it all over to inches at that stage and moving our decimal place (which we need to do to properly add it it to the wheel diameter which is given in inches). Technically, the aspect ratio is a percentage of the section width (or 75 % in our example). It's easier to just use a whole number and use 2540 in the conversion part. (instead of .75 and 25.40)

Now you take this converted number, which is the height in inches of the two sidewalls, and add it to the rim diameter (or hole in the middle of the tire).

You now have the approximate diameter of the tire in inches. It should work well enough to figure out your clearances, etc.

Thank you very much Bobby....I knew that you would have the "Proper" answer.