I ran across this in a cyclingabout article: "Friction Facts have tested a singlespeed chain to be about 99% efficient at 150 watts (2-watts drag).... " Elsewhere, I think in one of the articles cited above, a graph by Ceramic Bearing, showed Dura Ace to be about 96.5% efficient at 150 watts. So this would imply that SS is about 2.5%, or about 4 watts better than Dura Ace. Doesn't sound like much and I am sure if the cadence goes very high or very low any gain in efficiency for the SS is quickly lost. Elsewhere I have read that a derailleur drivetrain can have efficiency as low as 88%. That would definitely make a difference.