On a recent long, hot journey in Spain, I was pondering which was the most effic
ID: 1378393 • Letter: O
Question
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.
Explanation / Answer
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.