Thanks prj71, I have seen both links. Neither one provides actual data on single speed vs geared. Intuitively, I would think single speed is more efficient mechanically, but is there a significant difference? I think we are pitting the efficiency of the human body at a reasonable cadence (geared) against the mechanical efficiency of SS. Of course, in hilly terrain and strong winds, I would think geared always wins. On flatter courses and light wind, the SS might win. I am interested because I sometimes go with a nearby informal race that is relatively flat. I am 75 and need all the help I can get (at least 20 years older than anyone else out there), so if SS would give me 2% or so that could be significant. Similarly for a time trialist.