[aerohead]

Where it's humid, and it can feel like 100-F, when it's 80-F, without AC, you almost wish there was no glass at all!
At 65-mph, by 'tank' mileage I've seen a 2-3 mpg loss with the side windows down, and slider rear window open.
Running the AC at the same speed meant around the same 2-3 mpg loss. So it was a 'wash.'
With modern heat pump systems, I suspect that its more efficient to keep all the windows up, and just run the AC. And on full-recirc where it's super humid.