How do you do it? Do you measure from edge to edge straight across? Sorry if this is a dumb question, but a buddy of mine was asking what size they are (we were looking for tires), and I wasn't sure if they were 17 or 18 on my manual Touring model. So I went out with a measuring tape and they measured 19"?

 

Just read the tire size, it's the easiest way.

It will say something like 235/40/18.

That's 225mm width, 40% aspect ratio (40% as high as it is wide) and 18" rim.

 

I see. Always talked about rim sizes, but never tried measuring them. Seemed like something pretty straight forward.

 

It will get pretty tough measuring them because of the difference in lip sizes from wheel manufacturer to manufacturer, they wont always be exactly the same in diameter. Like jeffrey said just read the tire itself and that will tell you.