Why would one ever buy a 12-25 cassette when an 11-25 is available? I was looking at purchasing a new cassette and chain. Looking at the available cassettes for SRAM components, there is a big variety of sizes available:


11-23: 11-12-13-14-15-16-17-19-21-23
11-25: 11-12-13-14-15-17-19-21-23-25
11-26: 11-12-13-14-15-17-19-21-23-26
11-28: 11-12-13-14-15-17-19-22-25-28
11-32: 11-12-13-15-17-19-22-25-28-32
12-25: 12-13-14-15-16-17-19-21-23-25
12-26: 12-13-14-15-16-17-19-21-23-26
12-27: 12-13-14-15-16-17-19-21-24-27
12-28: 12-13-14-15-16-17-19-22-25-28
12-32: 12-13-14-15-17-19-22-25-28-32


Now I can understand why someone would choose a cassette based off of how many teeth the big gear has. Some people live in hilly or mountainous areas and desire a higher number of teeth. Some people live in flat areas where they simply won't use those cassettes with a lot of teeth on the big gear.

But why would anyone ever choose to buy a cassette with 12 teeth on the small gear instead of 11? 11 teeth = higher maximum speed. I doubt many people buying a nice set of components (Shimano, SRAM etc) would ever say "I don't need to go faster so 12 teeth is fine with me."

What's the point of 12 teeth when you could opt for 11 instead?
If you want to maximize your max. speed, go for an 11 tooth cog. If you want to maximize your average speed, unless you're a pro you probably are better off without it. Even cruising at 40km/h does not require and 11 tooth cog.

For example, take a look at this table, showing cruising1 speeds for a 11- 21 tooth cassette:


And compare to this table for a 12 - 25 tooth cassette:



You can see that the 11 tooth cog is only really useful for cruising speeds that riders at the professional level can maintain, or for short bursts. So, like I said, if you aren't trying to break your own max speed record, you may be better off with a 12 tooth cog.

(Tables screen captured from bikecalc.com. Check it out.)

I remember reading a while back about how most riders can't maintain a high enough wattage output to make an 11 tooth cog worthwhile, but I couldn't turn it up. Maybe another contributor will post it.

1 Cruising here means pedalling at 90 r.p.m.