It's expensive to have the windows down.

[dududf]

why not use both A/C and having the window down?

*Hopes you're joking.
_____________________
Give the old guy a break, it may have applied for him when he was younger.

 

[Rasputin1]

I remember Top Gear did a test to see how much resistance from the wind actually affects fuel consumption.
They strapped one of those camping chairs to the top of the car, and ended up spending £12 or so extra on fuel(they made the trip twice).

So maybe that says something?