I asked this in a thread over in Recumbents and HPVs. There was a post about Schlumpf mountain drives, but looking at the website it seems you can get "Speed" drives as well. This is more a question for the engineering minded people: would using a High Speed drive with a 1:2.5 ratio or something like that have any benefits over a derailleur system? My first thought was that if it improved torque and efficiency, it could provide a bit of a boost in TTing even if you look like a prize fool. Fixed is another idea - could you conceivably use one to have two gears? I have no understanding of SA or Rohloff hubs either so really don't understand the benefits of planetary gearing other than the fact it is low maintenance.