I've been riding 165MM cranks on my TT bike for years, and the track bike (only a year) also has 165's. To be honest, when I went from 170's to 165's on the TT bike, I don't recall feeling a difference. Apparently there was one, though, because my times over a 12 or 20K course started to improve. My cadence also came up a bit, too, but I don't know that I'd attribute that directly to the cranks.