Ventilation / Wind Deflectors

So, after buying one of the pet security guards with the thought that a) it would fit in the front window and b) I could use as more secure ventilation, it turned out neither of these things worked.
So, instead, I bought some wind deflectors, with the idea that I can have the window open a little at night and a) passerby's won't be able to see that the window is open and b)adds a tad bit of security, as it's harder to get a hand in the window etc

Apparently, they also increase fuel efficiency as they reduce drag when the windows are open etc, however, I've not tested this as yet, so not sure how true it is!