Here is something I found that may be of use to some of you when trying to determine what size tires to buy, works for compairing any size tires, even pick up and car tires, just plug in the tire sizes and hit calculate.
http://www.tacomaworld.com/forum/tir...0r18-100-80r19

 

Not sure how exact you want to be, however, to obtain exact data you should use the manufactures published size for OD's and width's. That said, using Dunlops web page....question and re question their sizes as they fu bared me one time.

In short a 180x18 will differ as much as 3/8" in OD from one manufacture to another and in Dunlops case, one model to another will differ a lot.