I need higher gearing for the high way . I have changed my front sprocket to a 15 t. But this is as big as I can go with my housing . What can I do with the rear ????? What size to give me more top end . Can any one help . At the moment it is sitting on about 4200 rpm at 100km I would like to get it down to 3500 rpm can any one advise on this .