Not your normal wheel and tire question...

[Popeye]

How come no matter what country you are in, TIRES are in mm, both for width AND height? Yet the WHEELS are in INCHES?

[talonXracer]

I buy my tires in inch sizes,,,,,,31-10.5-15

But the "new" world standard for passenger tires is indeed metric sizes. That way the manufacturers only have to deal with a single size regardless of the market the vehicle is destined for.

[Zzyzx]

Side wall height is actually stated as a percentage/ratio between the side wall and section width, not as direct mm value.

AKA, 225/50-16 does not have the same side wall height as 205/50-16... the 225's side wall would measure ~112mm where as the side wall height of the 205 would be ~102

[Popeye]

Oh....
How is that calculated? Not that I NEED to know...

[Zzyzx]

Oh....
How is that calculated? Not that I NEED to know...

The 2nd number tells you the ratio between sidewall & section width.

So, a XXX/50-XX would say that the side wall height is 50% of the section width (that 1st XXX) and if it said XXX/40-XX then the side wall is 40% of the section width.. ect ect ect..

Real life example for this car:
Stock tire = 205/50/R16
that means the side wall is 50% of the section width, or 205*.5 = 102.5mm (~4 inches tall)
An up size tire of 225/45-16 would net 45% of the section width, so 225*.45 = 101.25 (~4 inches tall)

Makes watching people talk about "40"'s being too "low" or 50's being too "high" more entertaining....

[Popeye]

Well shit, that makes TOTAL sense!

[gtolio]

Tirerack has a great reference section for this kind of information here: http://www.tirerack.com/about/techcenter.jsp

Here is the article about tire sizing as it pertains to this thread: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46