A very long string of numbers is used to identify any given tire. Can anybody tell me what each of those numbers actually translates to in terms of the tire's dimensions? For example I assume that the Rxx designation is the inner diameter of the tire... is that true?

 

now i may be wrong on this one but i doubt it so here goes. say you have a 285 35 r18 the 285 is how many millimeters across the tread is, the 35 is how many mm from the edge of the rim to the edge of the tire and the 18 is the rim diameter (in inches).

 

You were almost right. Good job. He's right that the first number, i.e. 285, represents a metric measurement in mm. The second number however is an aspect ratio relating to the height of the tire sidewall. I don't have the formula handy, but it represents a percentage of the width. It is not a constant height. Therefore a 225/35/18 would have a much shorter sidewall than a 285/35/18. In lamen's terms, 35% of 225mm is smaller than 35% of 285mm. And of course the last number represents the diameter of the wheel.

In reference to the first number, it does not indicate exactly what the width of the contact patch will be. It is based on the fact that you will be using the appropriate rim width for that tire size. Basically, a 285 on an 8.5" width will have less contact with the road than a rim with a 10" width, that in turn stretches the tire negating that "balloon effect". That's all folks.

Good luck, Rishie
Automotive Research and Development

 

Thanks guys. That was exactly what I was looking for.