oh by the way, they way i figured it out was using this calculation
just imput your specific numbers and presto you got overall diameter
To find the Overall Diameter and Section Width of a 175/70R15 tire do the following:
1. Multiply 175 by .039 to convert the Section Width from millimeters to inches. 175 x .039 = 6.8".
2. You will need to find the Section Height to determine the Overall Diameter so multiply the Section Width by the Aspect Ratio.
6.8 x .70 = 4.8".
3. To find the Overall Diameter you need to twice add the Section Height and then add this sum to the Rim Diameter.
4.76 + 4.76 +15 = 24.5"