I've been googling on it, but couldn't find an answer yet (a controlled study), so let's see what you think:
How much more efficient is a close ratio cassette compared to a wide range one in a flatter course?
This new 1x wave makes me think about this often.
For example, a 12-25 cassette vs. an 11-42, considering you are riding between the 13-19t cogs, same chainring, bike, and speed range.
So the 12-25 has 1 tooth jumps, and the 11-42 has 2 tooth jumps. We have all been there: riding the flats with a cassette that has no 16t cog, the 17t is too light, 15t is too hard (same thing in a cassette that has no 12, 14, or 18t such as the 11-42 at other speeds).
So how much energy is being wasted for not being on the desired cog?
Should we bother having different cassettes to choose from, or just use the biggest one we ever need for every ride?
The gap between a 15t and a 17t is 13.3%, so if you are riding 15t at 90rpm and shift to 17t you are suddenly at 102rpm. If you had a 16t, that is a 6.6% gap and the same shift gives you a cadence of 96rpm, which is a much nicer gap IMO. These cadence numbers are just to illustrate, I'm not talking about the absolute number, but the gap in the shifts. Do small shifts give us a big benefit? Racers loved corncob cassettes in the past! Where has that love gone?
I see many riders moving to 1x, and getting a wide range cassette that matches the range theiy had before with 2x, as if the bigger gaps made no difference for them. And then some people do that for a TT bike with an 11-28 or 11-32, where I suppose efficient cadence makes the most difference. On the other hand, taking off the front der. and one of the chainrings saves a few watts in aerodynamics, could that offset the cadence efficiency for a TT setup?
In my experience, big gaps are annoying when the gradient or wind changes slowly, and I get very upset in those times when one gear is too hard and the other is too easy. That happens with my older 2x10 bike which is setup with 50/34x12-36, and I do use that low gear for some serious hills around here. In this case, for this bike, although I hate the gaps, I hate climbing at 50rpm even more, so I took the 12-25 cassette off and put this 12-36 in place with a MTB der. But other than getting upset by the big gaps, and "feeling" it might not be the best cadence, I wonder if that "wrong" cadence actually steels me some watts. Usually the first gap that I find tolerable is 17-19, but even then I miss the 18t cog sometimes. This feeling is paradoxical, because I also ride a fixie very often (49x17), with widely varied cadences, but in this case, when I don't have gears at all, I seem to switch off the cadence sensor in my brain and just ride the bike, my legs go CVT!
Still it pisses me off when I do shift a gear and the gap is too big...
What makes me waste neurons with this topic is that I'm thinking about a dedicated TT bike in the future, and for this use 1x does have an appeal, since TT (and triathlon, who knows what I might want in the future) courses are often flatter and/or shorter, so even if I have to do a short climb standing up, I might get that energy back from a small aero benefit over a longer time. Another reason for doing so is that I really want to experiment with shorter cranks, and I can find 135mm BMX cranks with 110bcd spider on ebay, but those will only accept one chainring unless I adapt the spider. Shorter cranks also help staying aero without loosing power. So all that makes me think about going 1x for a TT bike, using cassettes from 12-25 to 12-29 (campy). Any course requiring lighter gears, I'd do what I do nowadays, and put clips on my road bike.
For other applications such as road or gravel, I'm still a triple lover with 3x11, but q-factor also comes into play, and then that's a q-factor vs. gearing gaps discussion, possibly better left for another topic