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How to calculate tire sizes

stealthee

stealthee

New member
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

=SUM((((A1/25.4)*(B1*0.01)*2)+C1))

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.
 


So... Based on the example, you could run 275 60 14s and have the same sidewall ratio as your stock wheels.
225 55 17= 26.74409449
275 60 14= 26.99212598

I'm pretty sure 275s would rub though?
 
Probably would. This is where common sense has to come into play, though. Another factor is wheel offset. Some offsets allow you to run wider tires without rubbing. That's a completely different formula though.
 
Thank you for spelling this out for the people that didn't know. I have tried explaining this to many people in my lifetime that think the 55 means a wider tire if it is a lower number.

Regarding Spades comment, that 275 would rub because of the width, not because of diameter. The diameter on your example would be almost equal to stock like you calculated.
 
The misconeption comes from the way tire sizing used to be done. I remember getting into an argument about this subject with my dad when I was younger. He insisted that the lower the second number the wider the tire. It took him stopping at a local tire shop to be proven wrong, though he didn't want to admit it.
 


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