Thanks for all the methods posted. I'll have to try some of them and see if they are any easier than just using the old wind triangle method my dad taught me back in the sixties, but which requires an accurate Airspeed Indicator to calculate winds (or if I had an accurate wind speed I could calculate the Airspeed based on the GPS Ground Speed.)
Ultimately what I am trying to do is calculate the error in my airspeed indicator, which I'm fairly sure is off by about 20 or 30 MPH at the higher Indicated Air Speeds, but appears to be much closer to accurate at traffic pattern speeds. i.e. flying formation with a buddy, who "thinks his indicator is accurate", mine matches his at around 80 mph IAS, but indicates 170-175 mph when his indicates 145 mph. Since I'm powered by a VW-2180 I'm pretty sure I'm not really cruising at 180 MPH. (Although it feels like I am.) Todd Thelin
