Just bought the Kilo S3X from bikesdirect, I like it but the gear ratio (48:16) is way too low considering the gears only shift down from there. I believe the three gears are direct, 1:0.75 and 1:0.625. I have two questions:

1.) Can you put a 12 tooth cog on it? I'm not sure it will fit.
2.) Will a 12 tooth, on the middle gear (1:0.75) be roughly equivalent to how it is now on the direct gear, because 12 is 75% of 16?

Do you really need to go higher than 48-16?

https://sheldonbrown.com/gears/

The standard Sturmey Archer rear cog is 13T (the ones made and sold by SA). Supposedly there are other sizes available, but I have not seen them for sale. Shimano style cassette cogs fit, and I am using a Surly cog on one of my S3X builds.

46x13 is way too high for me, 42x13 is close to rideable. You must live in the flatlands.

So why do you want to go from a 13T cog to a 12T cog if the gearing is already too high for you? You are going the wrong way, which would explain seau's question of why you need to go higher than 80 gear inches for street use.

Is your area hilly? If so, try something around 65 gear inches. You can calculate gear inches by using this calculator:

https://software.bareknucklebrigade.c...it.applet.html

Just a note, while the S3X fits Shimano compatible splined cogs, Shimano brand single speed cogs will not fit. They aren't machined accurately enough slide on, go figure. Surly cogs work, but are more expensive.

The S3X hub is a fun way to ride fixed gear and it you don't have to maintain the same cadence under all conditions.

The S3X hub will let you ride harder and faster, and guarantees male orgasm.

Sturmey Archer S3X hub will never co k block you or steal your girlfriend.

Are you sure? Learn to spin and you'll appreciate it.

Spinning is for people who don't mash.

The Sturmey 12T S3X cog is part number HSL886 for 1/8 chain and HSL893 for 3/32 chain. You'll also need the special-for-12T HMW536 (2mm thick) or HMW535 (3mm thick) spacers.

48Tx12T on 700Cx23 wheels/tires will give you fixed gears of 66, 79 and 105 gear inches.

I'm very happy with 48x13 on mine. It gives me about 61, 73, 97 gear inches. 73 is a good cruising gear for flat ground. With 48x12, a 79 inch middle gear might be a little high.