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I built a single speed bike about a year ago, and am starting to change out parts to make it more comfortable for me. The first thing I'm wanting to change is the gear inches, which I didn't give much thought to at the time of riding. I have a 700x25c wheel with a 16t cog , and a 46T chainring that also has a 165mm crank arm. If I calculated the gear inches right, it's 79.2 in. I'm in a pretty hilly area, and I'm wanting to see what effect having a gear inch more around 70 in will have. I'm thinking of either getting a chainring that is 40 teeth or getting a cog that is 18 teeth. Is there any argument for which would be better to replace? Also, is there anything else I should be thinking about or considering when purchasing them?

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  • Is it correct that you would be replacing 16T cog with a 16T cog?
    – ojs
    Jul 19, 2018 at 12:11
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    It's generally easier/cheaper to replace the cog. Though of course you will need a longer chain. Jul 19, 2018 at 12:22
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    @DanielRHicks Looks like this is a single-speed or a fixie. The rear wheel should then be mounted in slots designed to allow for swapping of the rear cog with one of a different size without having to change the chain at all (within reason). Jul 19, 2018 at 12:29
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    @TheStrangeQuark the idea is that the chain is bent around a less tight curve, so each pivot of the chain then rotates through a lesser angle, causing less friction in the chain in every link. That's the idea, but I don't think it's enough to be a compelling answer to your question, so only worth a comment. I'm not even taking a view on it, just voicing the thought
    – Swifty
    Jul 19, 2018 at 16:13
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    I don't skid unless I really need to Jul 23, 2018 at 13:10

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For the same gear ratio, more teeth each end means:

  • More weight
  • Weaker chainring
  • More chain
  • Reduced angle at each chain pin as it wraps round the sprocket.
  • Potentially reduced vibration (this is complicated, I'll work on an edit later)
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    Also more teeth to spread force over, so they will wear out slower?
    – Robert
    Jul 24, 2018 at 0:05

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