im riding street with 26" wheels. perfect gearing for me (and maybe for quite a lot of people) would be about 2,1 - 2,2. so now you have 9t in a rear, so how much in the front? 19t? besides, chain have more abuse with micro gearing with generates more power. people snaping chains in few weeks after buying new ones. make it 11t and that be very good hub for 24" and 26" bikes.