Tom, are you thinking fixie or SS? I've got a flip-flop hub on my Mark V but I've only tried fixed a few times and wasn't really sold on the idea. I'm running 48x19, (checks the Sheldon Brown gear inch calculator) with my other equipment it works out to... 66.6 gear inches. Oooh! I have the gear inches of the beast!
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2009 Bike Friday Season Tikit (commuting folder)
2007 Rivendell Atlantis (touring, general riding, errand runner, stuff hauler)
2007 IRO Mark V (SS)
2006 Rockhopper Comp Disc (Icebike)