Anyone worked out which is more economical, having the air on or the sunroof open?

I don't have any numbers, but it will depend on the speed. At highway speeds(65 miles per hour and up), the drag of an opened sunroof(based on my instant MPG dial) uses up more gas than the air conditioning. But at lower speeds(around 40 MPH and lower) the sunroof and the AC break even.

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I'm not so sure it's this simple. The A/C uses an electric motor that will deplete your battery quicker and therefore make your engine run more often. The sunroof would add some drag, but I can't imagine it's that significant and most speeds. A better test would be to drive for a week with the sun roof open, then the next week use A/C. Assuming your driving habits and weather were similar, you would have a better test than the instantanous reading. Just my opinion

I guess you need to consider the amount of sunshine or rain in your decision too

Well, if you just run the vents versus the AC button, you don't use as much power. However, if the temperature outside is around 75, it isn't that comfortable.

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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.

You are correct about winds, also heavy rain.
My current tank WAS reading 5.7 litres per 100 K, tonight on the way home we had a thunderstorm which brought very strong winds and the road was awash. My eco drive reading never got beyond mid 7's and I watched the tank reading drop to 5.7, my commute in the rain was less than 25 miles.
The wind was mainly from the side but I noticed the car slow quickly when my foot was off the gas pedal which I'd guess had more to do with the water on the road than the wind. Tomorrow I have a 250 kilometre mainly highway trip in clear skies and temp around 68f, that should finish off this my first true full tank.