I took my GPS on a 200 mile drive this weekend after reading this thread. I had it on the entire time and compared the GPS speed with my speedometer reading. As long as I was moving, the speeds were with in .5 Mph all the time as all speeds. I also confirmed this by bringing along my radar gun and getting a reading of several over passes to get check the reading on my speedometer.

I have a Garmin GPS Map 76S but have not put it in a plane as of yet.

Just food for thought!

Enjoy,
Randy