Euro Black 4 Speedometer 30 Knot PitotPerimeter-lighted Black dial with bold White graphics, Black aluminum bezel, contoured White pointer and flat 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 Gauge - Hole size 3.4 86.36 mm