ishoot308, great explaination of the gps speed. I have seen in many situations that in a slight turn it will actually show a greater speed then the true speed. This is why when racers are trying to break speed records they have to complete two passes to ensure accuracy.
My gps speedo needs a new battery, but livorsi wants $200 to do the replacment. However, there is no law that I have to have a speedo on the boat. So frankly I don't know how fast or slow I am going. Would be very hard to make that stick in court.
