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I pick the highest gear that keeps me right over 2000 RPM. On the flats, that usually means 4th for 35 mph and 5th for 45 mph.
On one part of my commute, there's a slight downhill grade where I can put it in 6th at 45 mph. It's kind of low RPMs, but there's almost no engine load. Otherwise, I wouldn't do it.