Can someone please evaluate freewheeling single speed bicycles in terms of efficiency?

Answer 1

Don't really understand your question. A bicycle while freewheeling AKA coasting isn't receiving any energy input from the rider. W/o energy input there's really not much point in calculating efficency.

On a flat surface the bike will simply roll to a stop as air drag, bearing losses, and rolling resistance in the tires slowly use up all of the momentum or kinetic energy that was in the bike + rider at the moment the rider stopped pedalling.

If it's at a downhill slope, then gravity will try to drag the bike forward/down, if the frward vector of gravity is bigger than the bearing losses, rolling resistance etc etc.

On a singl speed bike while pedalling, the freewheel isn't moving WRT the hub, so the issue becomes moot. What you're left with is the efficiency of a chain drive( which is high, 98% or so). And for the whole vehicle you then have to subtract bearing losses, rolling resistance ASO.

If you include the rider you might want to consider the losses in turning food(chemical energy) into kinetic energ(i.e. the bike moving forward). And with that approach stuff like bearing losses aren't even on the chart and can be ignored.