Let me add this, as reality-based, but not scientific testimony.
I have a Garmin 440 plus a Tom Tom for the car. I've used the Tom Tom in the car, and it was always within 1 mph or dead even with the car's speedometer. In the boat, the Tom Tom was almost always dead even with the Garmin, as well as my Navionics App display on my iPhone. They were all pretty much dead even, within 1 mph max variation.
I'd have to say with your variation, either your car's speedo is off, or your GPS is somewhat screwed up. My boat speedo is pretty much dead on accurate up until 40 -45 mph. In the early stages, up to 25 or so, the GPS reads higher than my boat speedo. At 55 mph GPS, my boat speedo is bouncing around 57 or so, up t 60. After that? Well there is no after that, that's the max
