I have found that the a/c really does not have that much of an affect on the economy in reasonable temperatures. I have left the a/c in auto set to 73 with the eco on, and my mileage has been fairly decent over the last two tanks. Outside temps between 65 and 80.

Now what hurts my mileage the worst is the winds we have been having recently. I have been having a mix of strong head/side winds on my way home with very little tail wind on my drive to work. That has eaten my mileage from about 38 per tank to 34-35 per tank.