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When I had my 97 civic auto, I noticed that when at a red light and idling, if I turned on the AC, the engine rpms would move up a hair (thereby increasing fuel use. how much? I am not sure).
I thought the best time to use AC was on a downhill or coming to a stop. I used the AC as a 'brake' of sorts. Because if I'm cruising along, then coast with no throttle, turning on the AC would actually decelerate the car more than just coasting. With this thought, I figured the engine was not using more fuel with that method.
When I did my cross country trips, averaging 38mpg at 67mph cruising, I would do my best to only use AC on downhills or coasting, never on uphills or accelerating. It's hard to say, but I think this method helped. 38mpg was about the most I ever got in that car, including highway use in cooler temps with no AC use.
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