I think there is not enough length in the lesser linked chain (66) and not enough tension adjustment in the more linked chain (68) to work with the gear combos listed because of the gear diameters difference and the center to center distance constant. So if you could have a 67 link chain maid if should be possible to run the X combos. No binding or teeth jumping issues.Quick question? In the gearing chart why is there X`s in certain blocks. Is it ratios that shouldn`T be used due to chain binding or tooth jumping???????????????????