Click link for information on this problem and a machine that cleans up the system.1. The "recirculate" or "max a/c" setting will actually cause the a/c system to use LESS fuel and put less drag on the engine than the "normal a/c" or "fresh air" setting. In the "recirculate" or "max a/c" modes, the system recirculates the air in the vehicle, resulting in less operating time for the compressor, since with the a/c on the air inside the car will be a lot cooler than the air outside.
2. However, when using the "max a/c" or "recirculate" settings, take the a/c out of this mode and put it into the "fresh" or "normal a/c" setting a couple minutes before you stop driving. Even better, turn the a/c all the way off and set it to "vent" for a minute or so before you stop the car. This will dry off the a/c unit's evaporator and will prevent mold from growing on the evaporator. If mold grows, your a/c will put out a bad odor and may also get clogged up so it cools poorly.