I've seen this question asked countless times before and it seems the general rule is if you're traveling at highway speeds or above 45 MPH, it's more fuel efficient to use the A/C than to have the windows down. It doesn't specify how far the windows are rolled down. Are they rolled down completely or is it leaving a 1" gap?
For those of you with ScanGauges, do you notice less of a hit with the window partially rolled down 1" vs having the a/c on? I would figure less drag comes into play with the window only partially rolled down.
The A/C on with windows up is better than window open up to the point that there is no heat relief from window. On mildly warm days, I just roll them down at traffic lights and back up as I'm leaving the light.