How much more efficient is having the windows down as opposed to running the a/c? Because opening the windows increases drag from the wind coming in, decreasing FE. Another alternative is to have the vents open, but doesn't that increase drag as well? Anyone tried doing both and comparing FE? I tried searching the forums but found nothing specific in this regard..

Hello,
I have read that having the windows down with no a/c is about the same as having the a/c on and windows up. If you can, just run the vent if it's not too warm outside and leave your windows up.

This has been discussed in the forums before, but finding the thread might not be easy. From my recollection, the concensus (more or less) in the end was, less than 45 miles per hour, leave the windows down - the drag created won't be that great. 45-55 miles per hour, gray zone. Above 55 miles per hour, better off using air than leaving windows down.

btw, MythBusters did a segment on this awhile back. They ran quite a few tests around a track. Their conclusion? They didn't come up with one. Their tests were inconclusive...

KLCarch conducted an experiment on this topic that might be of interest to you . . .

http://www.greenhybrid.com/discuss/s...ht=windows+MPG

I do windows completely down at low speeds, windows partially down at mid speeds (40-60) and windows completely up at higher speeds with A/C on.

Windows partially up half way cuts down wind resistance quite a bit. My old accord picked up 2-5 mpg with the windows half way up compared to down.