Interesting. I only crack the windows anyway and mostly do city driving only.
I never knew it is lose-lose whichever you choose to do. I get over 30/gal w/o the a/c.
I burn gas much much faster if I use the A/C - losing 10 mi/gal. if my math is correct.
I don't have any numbers to add off the top of my head, but there is no way you loose 10miles/gallon from air conditioning. That's just not possible. It looks like 10% maximum which if you get 30miles/gallon w/o a/c on you would, at the very least, get 27miles/gallon w/ac.
EDIT - I found this online
Consumer Reports' auto-test department reports that the air conditioner reduces your car's fuel efficiency by up to 10 percent. So to achieve maximum fuel efficiency, motorists should avoid using the air conditioner at speeds below 40 mph and travel with their windows down, explains Gabe Shenhar, senior auto test engineer at Consumer Report's auto-test department.
"But as your speed increases to 45 mph, or highway speeds," says Jason Toews, co-founder of GasBuddy.com, "wind drag becomes an issue. Driving with the windows down increases the drag on your vehicle, resulting in decreased fuel economy by up to 10 percent. Drive at speeds over 55 mph with windows down and you'll decrease fuel economy by up to 20 percent or greater."