Stick your head under the console and check the back of the speedometer. You will find a black tube (no -- it's not an electrical cable). That tube runs from the speedo back to a pitot that can be in either of two places. 1) A plastic unit bolted to the transom or 2) the pitot on the lower unit of the outdrive. The most common causes of speedo failure are a kinked tube, a plugged pitot, disconnected pitot, or a pitot that has broken off or is flipped up. In operation, water pressure in the tube compresses air in the tube that unwinds a Bourdon tube linked to the speedo needle inside the speedo head.