[stefanv]

Hi everyone. First post in a long time ...

The common wisdom is that for city driving, and open window is more fuel efficient than A/C, and for highway, the opposite is true. I generally try to avoid using the A/C altogether, so usually just set the vent fan on high with the windows closed when on the highway.

Anyway, on one extremely hot day last week (in the midst of a run of very hot days), I felt the fan wasn't doing a very good job, so I cracked the window just slightly so the air already in the car had somewhere to go. This seemed to dramatically improve the ability of the fan to keep me cool.

I tried this for the next few days, and it doesn't seem to have had any measurable effect on fuel economy (my driving is mostly ~90km/h on country highways) as reported by Torque. It occurred to me that with just a very tiny gap of an opening, probably no air is coming IN to the car through the window. The positive pressure inside caused by the fan is blowing air OUT instead. What I feel when I hold my fingers near the gap seems to agree with this theory.

I haven't done any serious A-B-A testing of course, but my limited experience (a few days of driving compared to a almost a year of eco-driving before that), and my theoretical in-my-head analysis, suggests that this won't add any drag to the vehicle.

Anyone have any thoughts on the matter?