I have measured my existing seat post as 31.3 with vernier calipers, which should I buy to replace it, 30.9 or 31.6??
which seat post diameter
I'd measure the hole it's going in.
Look it up against the frame spec or read the markings on your existing frame, or measure the size of the orifice!
Far more likely to be undersized than over but best to check. What frame is it?
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