I didn't say all single-speeds are inefficient, I said my single-speed beach cruiser was inefficient for my commute on a regular basis. Guess the bike is geared too low, my legs go 'round 'n 'round real fast but my speed doesn't get any faster. I've done the same commute on a 20-something speed Fuji and and made better time with far less effort. Can't think of a better definition for efficient than making better time w/less effort.