Originally Posted by Mtracer

Your use of the term "sail-effect" suggest you believe the tailwind was pushing you or contributing a force that was helping you. It wasn't.

If you were going 27 MPH with a 20 MPH tailwind, you had a net 7 MPH air speed. There is no sail effect. You were not being pushed along by the tailwind. You were still working against a net 7 MPH air speed. Of course, a 7 MPH air speed is quite low. No question you weren't having to work very hard against it. But the tailwind was simply reducing what would have been a 27 MPH air speed to a 7 MPH air speed. Which of course does make for an enormous difference in aero drag.

The only time you're getting pushed by a tailwind is if your speed is less than the tailwind speed.