Yeah thats how I have always thought you figured it out, but I went on tirerack.com and they said to take the 12.8 and times by 50% and that will give you your height. And I did that and it just gave my height to the start of the tread, not the entire tire. So I guess it just depends on the tread added on. But I think that other formula 12.8x.60 works better, or gives you a more accurate number. If I did it by the 50% it only showed the 325/60/18 as about 30x12.8 so it only goes to the start of the tread. Thanks guys.