# How to calculate gear equivalents

Discussion in 'Cycling Equipment' started by androo, Sep 2, 2005.

1. ### androo New Member

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Hello,
My road bike has the a small chainring of 39 with a pinion of 28.
I am going to replace this with a 34 chainring. What I would like to know is what is the equivalent with the 34 ring ? Would it be the following 34X23 ?

My maths is hopeless , can anyone confirm this or point me to a site that has gear calculations.

regards
androo

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2. ### John M New Member

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39/28 comes up between a 34/24 and 34/25

Gear inches = chainring/rear cog X wheel diameter in inches (27" for most)

http://home.i1.net/~dwolfe/gerz/index.html

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5. ### Velo Steve New Member

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Here's another gear chart, which I think is easier and prettier (but I'm biased).

If you like you can put in just the two chainrings you are interested in (34 and 39). Choose a range of cogs (a.k.a. pinions) which suits you (say 21 to 28) and you'll immediately have a chart where you can see which combinations are about the same.

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