Some of you guys need to get out to play more!
I have a Garmin 430 in the aeroplane and a Garmin 296 portable that I use in the aeroplane, the car and the boat.
A few observations about GPS.
1) Steady state GS is very accurate and at 100 kph is spot on with the digital speedo in my car (GMH Commodore SV8) - this has been calibrated against a number of police radars - at some expense !!!!!!!
2) If you have a residual GS while stationary, your GPS is stuffed. I have ever never seen this. When stationary, my GPSs read 0 GS.
3) Although they say GPS is accurate to 35m in my experience it is closer to 1m. I regularly use the 296 to locate underwater structure when fishing. I can navigate to previously marked waypoints to within 1m.
Dr