Does Using Car Air Conditioning Reduce Mileage AT ALL?

Snopes has a good and educational post on car mileage and air conditioning. The air conditioner uses 5 to 30 horsepower, so it might reduce mileage by as much as 20%. On the other hand, driving with your windows down creates drag, which at highway speeds is likely worse for your mileage. The thread also reveals that the gas gauge is driven by manifold intake pressure, rather than direct measurement in the gas tank. The pressure is not all that accurate, apparently, but how inaccurate is not made clear.