Efficiency?

In the nice hot weather, is it obviously more efficient to either...

a) drive with the windows closed and the air-conditioning on
b) drive with the front windows down?

I am curious about this - I'll admit to preferring windows up and air-conditioning on (close to "LO") as the music I like to listen to is pretty embarrassing and I'd not want to inflict it on anyone else.

However, if it's obviously more efficient to drive around with the windows down (I guess fully or partially also makes a difference) then I might change my ways. I accept it might be a divvie question that everyone else knows the answer to... but wondered if it was a sure thing or open to debate.

Also, I know I could scientifically test this... but my digital range thingie seems to have a mind of its own... often it goes down drastically, often it stays at the same number for ages and, sometimes, while I'm driving along, it even goes up! I like it when it goes up...

Mythbusters tested this (twice in fact) and came to the conclusion that the point where you get more drag from having the windows down than the AC on is around 50mph. Mind you, that was an American car and so probably not that big on fuel efficiency. ;)

Ah, of course... it will be a function of speed.

Can I suppose that on a more efficient European car and mostly around 30mph I would be more 'efficient' with the windows down and my crap music blaring out for all to hear?