Originally Posted by
prathmann
When inflated the tires cross-section is fairly round. So in principle the 23mm tires would have an outside diameter of about 622mm (diameter of rim) + 2 x 23mm = 668mm or 26.3" whereas the 45mm tires would have an outside diameter of 712mm or 28". But in practice the actual widths vary some between different tire makes and models and it'll also vary a little depending on the width of the rim on which the tire is mounted
This will vary by width of rim used. Ever put a skinny road tire on a wider hybrid-type rim? You'll notice the tire lays much flatter and you can sometimes touch the rim to the ground when cornering sharply.