Electronic speedometers work by capturing pulses produced by a sensor. These pulses are electronically converted into a speed reading, much like a tachometer converts ignition pulses into RPM. Pitot tube speedometers operate using water pressure forced into the Pitot tube. This pressure is transmitted through flexible tubing to the moving Bourdon tube inside the speedometer head, where it is converted into a speed reading by the moving mechanism. Color: White. Hole: 4 in. Size: 3.375 in. x 3.346 in. Benefits: Wide speed range. Robust and reliable construction.