a 20 by 1.75 rim is 406mm actually 16 inches
a 20 by 1.3/8 and a 1.1/8 is 451mm 17 and three quarter inces
a sew up is 395 -15 inches inside and 432 =17inches outside
-its all about the added tyre profile to get to 20 inches
-stcik to the etro sizes
the funny thing is, that they fit bigger wheels to mini's and micro's, which i think is hilarious
although a 20 by 1.3, is the same rolling radius -with the tyre on, as a sew up
a 406 by 28mm etro tyre is a half inch smaller
-the 400 by 25 on the new redline micro, even smaller