Im still finding stuff out about my Civic. It seems it doesn't like lugging at all. If below 1500 rpm and no load at all your ok. But soon as there is any load its toast. The mpg below 1500 rpm loaded. Like trying to make it pull in 5th gear is as bad as being at 4000 rpms sailing the tach twords redline.
I am seeing the same thing with my accord, and my butt dyno. the car just doesnt like the rpms real low. I can go 30mph in 4th, and around 45 or so in 5th, but much lower it just doesnt have any torque...
I forgot to mention that my experimentation was with the AC on the latter half of the trip, and it's impact was barely noticed.
Consider that the AC simply can't sap as much power from the motor if rpms are low. Most AC compressors are designed to cool adequately at idle speed, and seem to run optimally just a couple hundred rpm higher than that, so any extra rpm you dump into it is pretty much wasted energy.
As a further experiment, I suggest a constant cruise rpm test of AC draw at low, mid, and high rpm. I'd be willing to bet that the losses are progressively and significantly higher with higher rpm.