Anyone know how to read racing slick sizes? For example, what does this tire size mean?

240/640R18

I think 240 refers to the width in mm, R18 I'm sure means it fits on an 18" wheel, but what in the world does 640 mean?

Thanks!

 

640 is the diameter of the tire in mm. 640mm tall.

 

Ah Hah! Why didn't I think of that? Seems obvious now, what the hell else would be 640mm lol.