Spun Silver 4 Speedometer 65 MPH PitotPerimeter-lighted Silver dial with bold Black graphics, polished stainless steel bezel, contoured Black pointer, and domed glass lens. 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.Pitot tube sold separately.4 - Hole Size 3.37