What is the methodology behind a standard speedometer?
A small rubber tube runs from the back of the speedometer to the boat's transom, where it passes through and is connected to a pressure-sensing fitting in the outdrive or a transom-mounted pitot (pick-up). In either case, forward movement of the boat results in changes in air pressure within the tube, which is 'translated' by the speedometer to boat speed in miles per hour. Speedometer accuracy can be influenced by many factors, including damage to the pitot tube and/or obstructions created by weeds, mud, or debris. For more accurate speed readings, use a GPS or a radar gun.