Inefficiency Comparison of Car Air Conditioning vs. Open Windows On a recent long, hot journey in Spain, I was pondering which was the most efficient way of cooling the car.  Which of these would be the most effective?


*

*Switching on the air conditioning, thereby consuming more fuel

*Winding down the (both driver and passenger) windows, thus disrupting the airflow around the car and ostensibly creating extra drag


Let's assume I'm travelling at 100km/h on level road, in a 1.2 litre Fiat 500.  (It's a valid assumption, because it was true).  Let's also assume that both methods cool me down equally as well.
For what's worth, I went with option 2.
 A: That is a rather difficult calculation, even if we assume that both processes are equally effective at cooling.  You would need to know the drag coefficient of the car with windows up and down as well as the efficiency of the AC.  The best way to settle a question of this sort is with an empirical experiment.  
It turns out that this is precisely what the Mythbusters did, as reported in this post.  They found that driving with the windows down significantly beat driving with the AC at full blast, getting 24 kilometers more out of the windows down car.  They were driving at 72 kph, which is a little slower than your case.
A: Let´s assume... it´s not valid at all. It depends completelly on many factors: interior T, exterior T, performance of your AC equipment, your "feeling"... From my point of view, of course you can reach the outside T of the car, which could be -10°C if you want (in winter)... and your AC won´t be able to reach that T for sure. 
But in summer, with 30°C outside, there´s no way for you to have better results by opening the windows: you need to turn on your AC, so spend some money in gas, if you want to have less than 30°C in your car.
For your information, cars are equipped with AC capable to reach a confort T (let´s say around 25°C in heads). But that´s a little bit tricky too: Many people prefers to "feel" more than that or more, so they control the AC in different ways. You can only control the epperture of windows, more o less, but you´ll never be below the out T.
Just an opinion, but your question and assumption is kind of lazy. Best regards,
