Originally Posted by
fietsbob
NB: 30.6 is the metric equivalent to an inch and a half..
No. 30.6 mm = 1.20". 1-1/2" = 38.1 mm. I expect the closest common seat collar size that will work for the OP is 30.0 mm. The next common size up is 1-1/4" = 31.8 mm.