Originally Posted by
PeteHski
I think the differences in friction between practical sprocket sizes sometimes gets blown out of proportion. The entire drivetrain typically has a power transmission efficiency of around 96%, give or take 1%. So at 200W you could expect to lose around 6-10W max depending on how clean your drivetrain is. While that is still significant, there isn't a lot you can do about it. For example a 2x drivetrain might save you a couple of Watts over an equivalent 1x due to chain line differences, gear sizes etc. Basically not very much. Then when you start looking at differences between things like 10t vs 11t sprocket or a 52t vs 48t chainring on the same drivetrain, it's going to be fractions of Watts that really don't matter at all. Basically you stand to gain more from cleaning your drivetrain!
The range in drivetrain efficiency for derailleur bikes can be much larger than that; even more if the drivetrain is dirty. For single-speed track bikes, we've measured efficiency in the range of 98%; for triple-ring MTB drivetrains we've seen efficiencies in the range of low 90's up to 96%, depending on the ring-cog combination. If you're putting out 250 watts at the crank on a steep hill, that's like 10 watts difference, which is, um, a drag.
When we do aero testing on the road, the differences in drivetrain losses are large enough to be annoying and affect the precision of our estimates.