My Gemini has 1.5 but I use a 1 1/8 tube, hence the need to emply reducers to make it fit. I wonder if 1.5 is a solution to a problem that did not exist?
Apart from the Lefty strut (which I believe is 1.5) I don't know if anyone else makes 1.5 tubes as yet?
Or is it down to marketing?