The speedo isn't part of the outdrive. There should be a device called a pitot tube mounted at the bottom edge of the transom that has a small tube running from it to the speedo. It works by water pushing into the pitot tube and compressing the air in the tubing. This change in pressure is what causes the speedo needle to move. If it isn't working, it could be that the pitot or tubing is plugged, or the tubing is split, kinked, or disconnected. Here's a pic of the pitot tube (pickup) : http://www.iboats.com/Pitot-Tube-Ea...5402826--**********.762428854--view_id.133574
Edit: The thread was moved/changed as I was replying so I hope this ends up in the right place.
