Thinking of switching off the cool air to save money? Ridiculous!
Potential Fuel Saving: $1.75/hr, maximum
Driving with the air conditioning off in the middle of summer is intensely unpleasant, uncomfortable, irrelevant and stupid. I know this because I've done it now for two straight weeks in ambient temperatures of more than 40 degrees C. (This was one of the 'hyper-miling' techniques we used to win the SUV class of the Global Green Challenge in 2009. Another was put all the windows up.)
For daily driving, however, if it’s hot outside, use the air-con. Even working its little heart out, mid-summer, an air conditioning system drains only around three kilowatts of power. And a decent petrol engine consumes around 300 grams of petrol per kilowatt, per hour. Three kilowatts for one hour of cool air means around 900 grams of petrol (about 1.3 litres). Call it $1.75 per hour, max. About the same cost as stopping every two hours for a bad coffee. (Less, if it's not working at maximum capacity.)