It's not just the A/C
 

I just did some testing with the SG today, and boy should have I done that before...

I found out that as soon as I switch the ventilation to any setting but off, the GPH usage jumps by at least 0.08. If I switch the fan speed to 4 and use the "max A/C" setting it's 0.1 GPH more.

And I spent all winter with strictly ventilation ON at fan speed 1 or 2 cause I thought 3 or 4 with A/C defrost would significantly use more fuel....

I'm a bit surprised how much fuel is used by the ventilation system. I was strolling along on a flat highway doing 45 (I can normally never drive that speed, but it was 2:30 PM, and the impact of accessories shows more at low loads) and was using roughly 0.85 GPH (53 mpg), I would set the ventilation to 1 which is barely noticeable and boom I'm at more or less 0.93 GPH (48 mpg). That's a good 10% drop in FE just by turning the ventilation on.

BTW, I could feel the car "hit" something when I turned on the ventilation doing 45 in 5th.

I've been seriously toying with the idea of putting a relay inline with the signal to engage the A/C clutch but I'm not sure it is an issue or not yet as I dont have the automatic climate contol. From what I keep seeing on various sites it is.

Are you sure you're a/c isn't turning on?

But ya, it takes quite a bit of energy to move air :D

Repeat with windows rolled down @10%, 50%, and 100% please :D

.08 vs .1...hmmm...defrost?

the bigger the engine, the less impact accessories take on your FE, but your FE isn't good to start with, so it doesn't matter as much. smaller engine is better. This is why the new EPA standards hit the small cars hard and the big ones not so much. more accounting of A/C usage and stress on the engine.