... And there's no doubt clean, well lubricated chains are efficient, though anything beyond that is a bit speculative. Testing showed that a Rohloff, with a standard lubricated chain was 94% efficient in direct drive. I'm not suggesting that can't be improved, but there isn't a lot to go at! ...
I recall reading some statistics on different drive train efficiency performed in a laboratory several years ago. For that particular testing they used a new Rohloff hub, not one that had been ridden for several thousand km to break it in. I am quite sure that my 12 year old Rohloff is now much more efficient now than it was new, both from polishing off the gears and also seals that have seen a lot of rotation over those years.
That said, I am sure many of you use your Rohloff hub(s) much more than I do, most of my riding is on derailleur bikes. I built up the Rohloff bike for heavy duty touring and have not done a tour with it since pre-covid. Thus, I am sure many of you have more efficient hubs than mine.
And there are so many different chain lubes with varying efficiency, that it is hard to make a good comparison. That said, lab tests should use the same lube between all tests when using chains.
I recall reading an article about six years ago comparing two different derailleur drive trains, a 1X against a 2X system. And they measured friction loses in each gear. The gears with larger sprockets and chainrings had less friction because as the chain leaves the sprocket and engages the chainring, the change of angle at the chain pins and chain plates was less angle with larger chainrings and sprockets. And better chainline also had lower friction. Thus, in this case there was huge variation from one gear to the next. Fortunately, I saved the graphic from the article to my hard drive, attached it. I saved this to my hard drive in May 2019, but I do not recall which publication I pulled that from.
Referring to that plot, what really surprises me is how many people use a 1X system for racing on courses like the Great Divide when that plot suggests that they would have much less wattage loss with a 2X system when they are in the higher gears. But, in this case a lot of people are racing with higher rolling resistance.
https://bikepacking.com/bikes/rigs-of-the-2024-tour-divide-stats/My point is that when there is this much variation in friction from gear to gear, it is pretty hard to aggregate that into a single number for broad assumptions.
In 2014 as I was finishing up my bike tour in Iceland, I met someone that had converted his Rohloff to belt. He was traveling with three other friends. He said that the belt added some friction, the way he knew that was because his friends were a bit faster than he was after the conversion than before he converted to belt.
Also, when I turn on my dyno powered lights, I think I am adding about 5 or 6 watts of rolling resistance, but I am unable to feel the difference when doing that. And when riding with friends so I can compare my speed and effort with theirs, it does not seem to change my rolling resistance at all if my lights are on or off. My assumption on drag with a dynohub is based on the first graphic at this link:
https://www.cyclinguk.org/sites/default/files/document/migrated/publication/feature-hub-dynamos.pdfThe date on that link is quite old, but dynohub designs have not changed much. Most of my hubs are SP PV8 hubs.
I usually leave my dynohub lights turned on unless I am instead using the hub to charge batteries, I do not consider that extra drag when deciding if I should have my lights on or off. And if I was going to get that concerned about drag when using the hub to charge batteries, I would charge batteries more often on the electric mains instead of from my muscles.
For me, the reduced maintenance for a belt is not a consideration. I worked in a bike shop before I went to college, most of my bikes are ones I built up from parts. I do not mind doing maintenance on a bike at all. But I know people that detest doing bike maintenance, I know an attorney (I think you call them solicitors in UK) that went into a bike shop and said he wanted the closest thing there was to a maintenance free bike. They sold him one with a Shimano IGH. He is very happy with it. Every couple years he brings it into a bike shop and asks them to do whatever it needs. The only maintenance he does is adding air to the tires.
