more than likely the pitot tube hole located on the leading edge of the lower unit, very small hole, is plugged with wood or debris. get a small drill bit or bent open hook and clean it out. use the drill by by hand and be careful not to break it off.
Or leave it clogged and get you a GPS. On the 3 boats I have owned the Speedo just made me feel better cause they all read high. My current boat says I am doing 72-73 when gps says 68.
