Chesapeake Black SS 4 Speedometer - 80MPH (Mechanical) 4 - Hole Size: 3.375 (85mm)
Electronic speedometers operate by capturing pulses produced by a sensor. the pulses are then electronically converted to a speed reading very much like a tachometer converts ignition pulses to RPM.
Pitot tube type speedometers operate by pressure from the water being forced into the pitot tube. This pressure is then transmitted through flexible tubing to the bourdon tube movement inside the speedometer head where it is converted into a speed reading by the movement mechanism.