It likely varies a lot based on the car, especially with hybrids. Also, IIRC they tested with all windows all the way down on the highway, which is excessive. I have them maybe 1/4 of the way down when driving 50+ mph.
Another case of OCD. Do whatever you are comfortable with and don't worry about it. The difference likely will be very small. Or.......run your own tests......over several weeks and several tanks of gas. Keep very accurate records of possible skewing factors; temperature, wind, elevation.....etc. and of the gas used. Then pay a major university to analyze the data for you.
I opt for windows open most of the time. One thing to be cognisant of though: it's good to run the A/C periodically, say at least monthly, to circulate oil in the lines, keeping the seals from drying out.
^ Not sure what you're asking. I opt for windows down instead of A/C in hot weather, especially for lower speeds and short trips, when increased drag is not an issue, and it's not going to be that effective anyway. On extended highway runs yeah turn on the A/C. That is unless we're with our younger son, who hates A/C. I do try to run the A/C at least a few minutes every so often, year 'round. Think I actually use A/C more in winter, for dehumidifying.
