The AC effect

I think you hit the nail on the head with the too many variables statement. I believe it's generally accepted that the AC's effect on mileage is less than leaving the windows down.
There is an effect obviously, I think it could be calculated if the amperage draw of the AC were known, but I think it's a variable speed compressor and therefore variable amperage draw.
And then there are of course other considerations, like I assume the traction battery doesn't like high temps and I assume the AC has some effect on cooling it?

 

What is the cost of cleaning sweat stains out of the seats?

Windows down? My dad had 2 hearing aids because he drove with the windows down.

 

My guess is the effect is close to 2 mpg. However, the reason why I am no longer using the AC is because we have cooler weather and that alone must have some effect on mpg too, so to make a real test you need to drive with the AC on and off at the same temperature and that is something I am not going to do...

 

USA is a little vague. I live in the Mississippi Delta, all summer I got 42 MPG, (95 degrees, high humidity) now I am getting 47 MPG in 60 to 70 degree temps.

 

My average MPG seems to be about the same in hot or cold temps. So there might also be some overhead running the heater, as well as the compressor running with the defroster.