[LarThaL]

Check out the pics below. These are all top rated max performance tires. Very revealing:

#1


What you see here, is that the 275 Michelin (left) is about 3mm narrower than the 265 Continental (right)


#2


The 265 Continental (left) is 10mm narrower than the 255 Yokohama (right)


#3


Finally, the 275 Michelin (left) is a full 13mm narrower than the 255 Yokohama (right)

I knew there are variations in sizes but for a 275 of one brand to be 13mm NARROWER than a 255 of another brand is quite surprising to me. This can be quite an issue with tight/aggressive fitments, rubbing, etc. Guess you really can't compare between tire brands. And then when you consider aspect ratio, this may affect overall diameter also.