Guys, I use my GPS as well, and yes it is SOG (speed over ground)... but I have an observation...
GPS is great for pinpoint location, of hand down to with 30 feet in any direction (including elevation). GPS CAN be more accurate, using various other services, and some of the newer/more expensive GPSses use them.
Here is what I am getting at. I don't think GPS is nearly accurate enough (with a basic handheld) to give you accurate enough numbers for speed. Yes I use it too. I think speed is less reliant upon location accuracy but, at these slow of speeds, I dunno how accurate it is.
Has anyone checked SOG of say a standard cheapo GPS handheld with a knotmeter to see how close they are?
As others have said, boat waterline (esp while heeling) changes thus changing max theoretical hull speed.
PS: My max hull speed is theoretically 5.99knots... my GPS has locked me at 6.5 on a reach. Again average over 2 minutes.