I was thinking the exact same question today! I've done my usual 100 mile weekend journey today and normally I get 32mpg crusing about 80mph along the motorway, today with the air con on the second notch (I'm not posh enough to have climate) it was down to 30.2, but that could be to do with anything. It was certainly lovely though in my car, much better than having your window down when yoru doing 80 like I saw a few older cars having to do!
I was told once, but don't know how true this is, that it's more economical to have air con running on the motorway than it is to have the windows down because of the drag you create by having them down, if anyone wants to explain why this wouldn't be true or why it might be true, then please, feel free!!![*-)]