Originally Posted by Polwart
the yam gauge is measuring speed through the water (probably as estimated from the vacuum generated by dragging a tube through the water)
Actually, I think it's pressure generated by having the open end of the tube facing forwards. The opening is usually on the leading edge of the lower unit; you can find it by running a fingernail up the front edge. May be somewhat clogged as well, which will add to the discrepancy between analog and electronic meters. [Note: If the tube falls off the guage, you get water spraying around inside the console. Make sure it's firmly affixed if you like your console dry.]
There are also aftermarket pitot speed guages; they usually have a triangular plastic pickup mounted on the transom.