In the real World, for the last 40 years, my teaching and practice, in
UK/NW Europe conditions, has been: default setting 1 knot, if you're
getting low or it doesn't look good ahead set 0 knots, if it's a really
good day and you're high and going well set 2 knots, if it's a fantastic
day, the best you've ever flown on and you look like setting a record, set
3 knots. If you are tempted to set 4 knots, land, take 2 asperin and lie
down, you're feverish. For strong thermal areas like the American South
West, South Africa and Australia, add one knot to all the above, except
perhaps the 0 knot case.



You go faster flying between 6 knot thermals with 2 knots set than you do
flying between 4 knot thermals with 4 knots set. The lower setting you
use, the greater your range and the greater your chance of finding the
really good thermals.


This reflects current practice, and it's really interesting. "Range" doesn't really describe the reason, I think. In the southwest USA at 10,000' we have tons of range, we don't stop for under 6 knots, yet flying at 100 knots does better than 120 (go look up mc 6 speed!), and 95 won't kill you.

I think the reason is more that flying at warp speed, you (or at least I) lose the feel of the air; I can't adjust slightly to benefit from gliding through rising air. And, I might fly right through that great thermal and not feel it.

John Cochrane