I have bought 2 "paddle-wheel" speedometers for 2 different boats. The packages included directions on how to accurise them. Basically you needed a measured distance so you could "clock" the boat and adjust the gear accordingly. There is probably a similiar requirement for "pitot tube" versions.
I doubt most factory boats have this procedure performed on them. Speed precision apparently was not considerred that critical since speedo/GPS discrepancy is occasionally discussed on other boat forums I visit.
If you want to accurise your instrument it is simpler to use a GPS than a measured course. During a calm period (no wind or chop) operate your boat at various speeds on a lake or large pond. Compare the readings. Adjust your speedo according to the directions. Repeat the procedure until you are comfortable with the results.
If you don't have, and cannot get, directions for your version examine the back of it for calibration access. Make a note of the current settings if possible. Then expiriment to see what makes it show higher or lower.
If you are not comfortable working on these systems then make a mental note or written table of the discrepancy at various speeds.
That's all I think I know about that.
Good luck!
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