Bought a gps/sonar combo at Bass Pro kind of on a whim and it's a problem. I didn't have a ton of money for a high end device, but I need speed for trolling coldwater fish, so I got a Humminbird 581ci HD DI. Seemed to be a great unit just no color but it appears as if this GPS is horribly inaccurate at slow speeds. I have a 20 year old handheld Garmin GPS IV and it's spot on, but this unit jumps all over the place. It becomes accurate over 3-4 mph, but I troll hardware at 2.2-2.4mph. Really disappointed but I already installed it on my boat. I might as well save the $300 and keep using my Piranha Max and my Garmin IV until I can afford a higher end unit. Reading reviews on the 581 it seems that mine is not defective, it is just the nature of this model. Even my old Droid cellphone is spot on with GPS speeds when using the Navionics app. So now that this is installed, I'm sure I have no options with BPS? They won't take it back, will they? This would be a great unit for someone who doesn't troll coldwater fish, the down imaging is awesome and shows structure great.