Fuel usage and AC
There are a lot of urban myths about fuel consumption when using the air conditioning in the car, some are silly, some are even ludicrous but unfortunately there seems to have been very little sensible scientific investigation on this subject. Certainly to keep cool in a hot car on a warm day it is certain that the car will use some more petrol to cover a set distance than it would in cold weather, maybe only a little but in certain circumstances an appreciable percentage.
If the car is travelling only slowly, say 30 mph then opening the window might be the most economical option but once the speed rises to say 50 mph or more then you would almost certainly save fuel by shutting all the windows and turning on the aircon. Initially any computer generated mpg figure might look very poor but within a few minutes as the car is cooled to the set temperature the estimated mpg figure will rise to near the level it was before the AC was switched on.
If you are concerned with your fuel economy when trying to keep cool there is a longer explanation of this on the website www.airconditioningforcars.co.uk/ACpage05.htm. Reading this will show you that sometimes keeping the windows shut and the AC left switched ON will be more economical than opening a window. Of course if the problem is a slightly misted windscreen then the safety function should easily over-ride any slight economy concerns - it would be dreadful to hit a pedestrian you had never noticed just because the screen was not completely clear.