An old Post from fish hawk
Typically the mechanical surface speed will show faster than your probe speed and SOG due to the fact that the water moving under your hull is accelerating as it travels the length of the boat. You can opt to calibrate that if you wish, or you can leave it at factory zero. Transducer location plays a role also.
Mechanical speed as we use it is not absolute, but is 100% repeatable. Using your probe speed as a reference point, it doesn't matter if the number is 2.0 or 200 - knowing what the probe speed is at the time of the strike and then duplicating that number is the important thing. "