MrBear..
When u off aircon, u' sure open your window/sunroof. So when u drive, the air gets into your car, hence more dragg. Try off your aircon, and close all your window, you'll feel the car runs smoother (prelude aerodynamic) compare if you off aircon and open your window (more drag due to air rushing into your cabin)
On the other hand, when u on aircon, if your aircon compressor is good, then it wont "tarik" that much power from your engine. But if your aircon compressor is no good or old already, then it'll tarik more hp from your engine <-- I kena this case b4. After I replace the aircon compressor, fc is much better.
In your case, i think your aircon compressor is in good condition la :)
Either way, the best way to save fuel is to off aircon, maintain throttle level less than 30% (more or less about 1000-2500 rpm - fuel injection maintain at "closed loop" rather than "open loop"), and make sure windows open cukup-cukup only for you to breath and not feel hot.
Just my thoughts.