gps speed is very accurate but may vary from a speedometer that uses a paddle wheel. I use to have a depthfinder with the paddle wheel that showed me going 3 mph but my gps showed me going 2 mph. so now I just use my gps. and in the summer months I like to run about 1.8 to 2.0 mph. but there are a lot of guys that troll even slower by there gps. this is only my opinion and others may disagree.