How does air conditioning waste gas? If in fact it really does?

The generally held conception is that driving over 40 mph with the windows up and a/c on is wasting no more gas than running with all windows down, true for the most part. The a/c works by using a drive belt to turn a compressor mounted on the engine connected by a pulley. This pulley offers high resistance to the belt trying to turn it. The engine must then work harder to overcome that resistance just to keep it spinning normally. Think about it this way - let's say you burn 3 calories a minute. You break into a run. You are then burning more than 3 calories a minute. Why? Because your body is working harder because it's more of an effort than walking. So the a/c is making your engine "run" instead of walk.