Originally Posted by
noglider
OK, so maybe I shouldn't use a dynamo hub on a century ride if I'm not feeling confident about my ability compared with my riding companions. But for commuting and errands, it's nothing to worry about.
That sounds about right to me. Only worry about it when that 5 - 7% might matter like when you plan to be riding near max speed for extended periods or on a distance ride that will be near your limits. Otherwise think of it as training resistance to build your endurance faster!
Some of the better dynos I think might even be in the 60% range for mechanical efficiency.