If it's speed relative to the earth, It's true speed. As in true wind and apparent wind.

I'm no authority here on this, but to WHAT DECIMEL point are you talking about when you are talking about accuracy? Are car speedometers accurate? Sometimes, put different diameter tires on your truck, need to change the settings in the computer on board, if it's an OBDII vehicle.

For all practical purposes, within reason, a standard GPS seems pretty accurate, especially, as has been stated, when looking at average speed.

Again, to what decimal point do you want the accuracy?