Windows on the highway and grill block

[GasSavers_BIBI]

I was wondering what would be the best thing to do for aerodinamics with my civic 2 door hatchback. I don't know if the fact that I only have two windows change something, but I would like to know if I can let my two windows open about a inch when I'm driving on the highway, or it is really recommended to closed them both. What are they impact?

And in winter, I usually let my windows open about a inch, half an inch, will that do a litle effect on my MPG or sould I just closed it?

About the grill, what material should I use to block it, and I saw people letting some places opens. I'm driving in winter and in snow to. Could people that have there grill give me some clue plz.

[brucepick]

Summer highway windows
All glass shut should give the best FE. However sanity has to prevail; we all have our limits as to how uncomfortable we're willing to be. Usually vents open plus some combination of windows cracked will make your ride reasonably comfortable. That will have lower FE than all windows shut but will be better than all open or AC on with windows shut.

Winter windows
OK, this being July that will be the topic of least interest by now. But here goes...

With windows open in winter there is more air going through the heat exchanger which will draw more heat off the engine. You'll be constantly drawing heat off the engine which it has to replace. I keep my windows shut, and have plans to block off the passenger side duct next winter for the many times when there is no passenger in my car.

Presumably all cars today have some flow-through ventilation so you won't asphyxiate while driving.