4″ Speedometer (65 MPH) Pitot – Spun Silver 4″ – Hole Size: 3.375″ (85mm) 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. Benefits: Wide range of speeds Rugged, reliable construction