I agree with Bruce and Evan. Taking off with a tailwind is similar to flying at a higher density altitude; anyway, the higher the altitude the more space between air molecules therefore the groundspeed of an aircraft is faster than the indicated airspeed. The opposite occurs when landing into a 10 mph headwind. Subtract 10 mph from your indicated airspeed and you have a good idea of your ground speed. I suppose that a pilot could whip out the old E6B and figure it out quite precisely if needed. I'm trying to figure out how to hang on to my E6B and my transmitter and fly and calculate at the same time. These forums are really great, they get me to really thinking. Always a student pilot. Chet