The newer GPS units will display the speed to 0.1 mph, or at least my Garmin does. As far as the easiest way I know to slow your boat down and cheapest a friend of mine did to slow his boat down. He took a 5 gallon bucket cut a 5" diameter hole in it attached it to a rope and ties it off to the boat. This slowed him down to the speed he wanted. After he tried that and it worked for what he wanted he purchased a drift sock and uses it now.