For solo rides on our modest terrain (flats and modest hills) I don’t see a lot of net difference in terms of average speed between SS and geared.

Mechanical efficiency differences are probably a few watts. A bigger difference will be differences in cadence. Formenti at King’s College has published a couple of papers in the last decade documenting the internal work we do cycling as a function of cadence. This is the work we do moving our legs, feet and the cranks and pedals around in a circle. Think of it as the overhead we have to do to keep the feet in a position to push the pedals.

It’s a sharply increasing function of cadence and reaches 1 Watt per kg (of rider mass) at 110 rpm.

So on a hill climb, SS is likely to make you a bit more efficient at a lower cadence but at the cost of much higher pedal force and muscle strain.

Conversely, if you spin super fast to go a bit faster downhill or to keep up in a pace group, SS will make you less efficient than a similar geared rider able to hold that speed with a reasonable cadence.

If you are trying to keep up with geared riders, you should probably have gears, too. For solo, rides, it makes less difference than you’d think.

Otto