Once upon a time, I did an experiment with my car and determined that it didn't really matter if I blew my a/c on high or low, it all came out about the same: using the lowest setting would only save me about 90 cents a year. I think I can afford that.
Lately I have experimented to see the difference between using the a/c versus not, as in, turning it off completely and rolling down the windows, or using just the fan. I only did the experiment for the duration of one tank of gas (it's really not enjoyable to go without air conditioning in Utah at this time of year), so there's obviously going to be a huge margin of error, but it's still interesting.
The first four months of this year I drove approximately 2,229.4 miles. Rounding to 2,230, I assumed a total-year mileage of 6690.
The average gas mileage for those first four months was 27.44 mpg. At that rate, I would need 243.80 gallons for the entire year, which, at a cost of $2.459 per gallon, would total $599.50.
The average gas mileage while using no a/c was 30.97 mpg. At that rate, I would need 216.02 gallons for the entire year, which, at a cost of $2.459 per gallon, would total $531.19.
Thus, I would save 27.78 gallons (my approximate milk consumption for nine weeks), and $68.31. That sounds pretty good, but then consider how that breaks down: I'm really only saving $5.69 a month. And that's for driving with NO air conditioning.
I think my comfort is worth $5.69 a month. Sorry to any environmentalists out there. I do kind of enjoy the fresh air at night or when it's cool, so at those times I may shut off the a/c to save some gas, and of course I'm still trying to cut down my car use entirely by using public transportation instead, but when I'm hot and I'm driving my car, I'm going to use the air conditioning. That is my conclusion.