XT speedos seem to be 8-10% over optimistic on a stock bike, so expect it to be somewhere in the region of a 15+% over-read with a 14T front sprocket.

Check it against a GPS. As long as you know the level of inaccuracy, then you can take it into account when reading/adjusting your speed. If it bothers you (or you don't like mental maths), then you can fit a Speedo-Healer.

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