I am sure this has been covered in the past, and I think even in my short time here I have posted the formula for calculating tire diameter, but since the question "Will this fit?" comes up so often I figured this deserves its own dedicated thread.
Stock tire size on my 07 GT is 225/55/17
225 is the section width in millimeters. To convert that to inches you would divide it by 25.4.
55 is percentage of diameter that is the sidewall height.
17 is, obviously, the rim diameter
The math formula to figure out the overall diameter would be the following:
((225/25.4) * .55) * 2) + 17 = 26.744
If you have Excel you can input the following formula
Place the tire size in A1, B1, and C1, and paste the fomula in D1
So 225 would be in A1, 55 in B1, 17 in C1 and the formula will automatically do the math for you. Since the sidwall height is based on the section width, if you go wider you will get taller unless you also change the sidewall ratio. A lot of people have trouble understaning this concept. You can use the formula to play with different widths and see how it affects the overall height.
Sure there are plenty of tire size calculators all over the internet, but if you have the formula saved on your desktop you can instantly see if your choice of wheel/tire combo will be too tall, or too short.