A basically pointless urban myth has arisen once more in the form of a Mobil Oil radio advertisement. The ad recommends that car drivers turn on their air conditioning rather than roll down their windows to stay cool. They claim that the A/C uses little energy while the open windows and rear window act like a huge parachute, making the engine work harder at a given speed.
Notwithstanding that few people roll down their windows completely and a bit of intuition should invalidate the claim, some basic consumer reporting debunks the advertisement completely.
So why does Mobil Oil do this kind of stuff? (1) Because they can, and (2) people lap this up because they don't have to feel inconvenienced while believing they have helped conserve energy.