Oh, to add yet another opine,
Since I don't commute, I train for racing--road/track/cyclocross, I ride the fixie for fitness. In that respect, riding flat roads on a fixie is a sorta a waste of time. Luckily here in vincino of Washington DC, it's somewhat hilly--great for fixie riding.
In Fall and Winter I go as low as a 39x16 to get my legs used to it and to develop a great spin (track racing requires it). Into January-Feb it's time for 42x16. When it's getting time to race, It's 48x16 and then 48x14 (race gearing) or 50x15. Match Sprinter Marty Nothstein uses 48x14. Of course it's personal--whether you are a masher or spinner and what the terrain is like where you live. At 43, I respect my knees and prefer spinning rather than mashing.
Giacomo Bellora, Falls Church, VA
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