Ok, I scanned through the stickies and didn't see this anywhere so I thought I would share.

I thought I understood more or less how modern tire sizing works. Every where I read says the first number is the metric width, the second number is the aspect ratio of the tire height and wheel height, and lastly the 3rd number is the wheels size. No real solid explination of how it really works.


I was watching a show on the tour of the Cooker tire plant and the guy from Cooker explained it like this, and it makes for more sense to me. The first number is the width in millimeters, the second number is the side wall thickness given in a percentage of the tire width, and the 3rd number is the wheels size.

So if my tire is a 285/75r18, it is 285 millimeters wide, the side wall is 75 percent of that or 213.75 millimeters. For a total tire height it would be the (sidewall [213.75 millimeters] x 2) + the wheel size.

Now from what I have been able to find, there is no federal regulation on tire size. So just because it lists a tire size, doesn't mean it will act be the correct size. So you should always reference the tire manufacturer's spec sheet for that tire to be 100% on the tire.

Hope that is clear as mud. Maybe someone else might find that to be a useful bit of info, or you can show of to your buddies when you can tell him what all that stuff means on the side of the tire.

 

Or this little guy is simple, if you do not have a calc on hand..

http://www.miata.net/garage/tirecalc.html

 

Not to say that I would break out a calculator and figure out what the size is the hard way. It is nice to really know how to do it now though.


That is a pretty neat calculator. Summit Racing has one that even figures in gear ratio's as well. I will have to see if I can find the link on my computer at home.

Edit: Never mind, it was a chart not a neat automatic calculator like you posted.