Interestingly, every time that little cassette cog got smaller, it was for the same reasons. For example, front shifting requires big changes to your pedaling cadence and breaks up the rhythm of your ride. These shifts cost rider efficiency and detract from the riding experience. Smaller cogs (like our X-Range 10 tooth cog) allow you to use smaller chainrings to get to the same top speed. If the largest cog on the cassette remains the same size, you can stay in the big ring longer and limit the number of front downshifts you have to make.
Put it to the Test
So, for a real apples-to-apples comparison, it's best to compare cogs on the same 12-speed drivetrain. Our internal testing showed that the drop in efficiency between the 11 and 10-tooth cogs came to 1%. To put that in perspective, on the same drivetrain, the drop in efficiency between the 12 and 11-tooth cogs is 0.87%. This means that the delta between the two comes out to just 0.13%. So, the reality is that efficiency doesn't fall off a cliff when riding in the 10-tooth cog.
With that said, admittedly this data doesn't fully resolve the question of efficiency between 11 and 10-tooth cogs, because in order to make an equivalent comparison the gear ratio should be the same between the two. To get the same gear ratio on a 10-tooth cog when using an 11-tooth cog, you need a chainring that is roughly five teeth larger. As we know, this larger chainring provides the efficiency benefit of lower chain tension in the upper drivetrain for the same load. But we’re looking at tenths of a percent in added efficiency due to that. This is because the larger ring increases the number of links required to drive each rotation of the rear wheel. More links rotating equals more friction, so the increased chainring size doesn’t have as much benefit as it seems. So with all else held equal, more chain links moving through the lower drivetrain (the low-tension section where the chain exits the chainring and then goes through the rear derailleur pulleys) incurs more losses, offsetting a portion of the benefit gained in the upper portion of the drivetrain from larger chainrings and cogs.
The point being, we're talking about very small numbers that are largely theoretical outside of a lab test. Out on the road, chain lubrication and cleanliness have a far greater effect on drivetrain efficiency than what can be gained by using an 11-tooth start cassette. Plus, with a traditional cassette, you miss out on other real-world time-saving benefits, like making fewer front shifts.