I've been driving w/left front and right rear windows down at speeds under 45 mph (although at times I exceed that speed), and the outside temp is under 80 F. I see no point in moving hot air through the car. In a low humidity environment, a temp of 90 feels like 90 whether moving or standing still, unlike a humid environment when movement seems to result in a feel of coolness.

I live in S. Cal. I grew up in Chicago, and ride a bike through E. Canada in the summer. Even on humid days, once moving, the air temp feels cooler.

So, for me the, "break even point" would be ambient temp loaded. In the evenings, as I did this evening, driving with the windows down did seem to net better milage, and a cool breeze through the car.