Don't overlook the fact that the A/C's job is to provide comfort to you AND your passengers. In other words, saving a few drops of fuel while the passengers at the back seat are sweating isn't right. :D There's no one fixed thermostat or fan setting that will be applicable to all situations. Adjust them accordingly so that all passengers are comfortable.

Keep in mind that whenever you or your passengers feel cold, you're using more gasoline than necessary. Adjust accordingly and for every situation.

In most cars, idle fuel consumption increases by about 50% whenever the A/C compressor is running. In my video below, with the A/C off the engine consumes about 0.88 liters per hour (LPH). When the A/C kicks in, the number jumps to 1.32 LPH.

[ame="http://www.youtube.com/watch?v=3KPF4NPrz2Q"]YouTube - 8th Gen Civic FD (R18 - A/T) idle fuel consumption[/ame]