[redpoint5]

No measured data to share, but my sense is you are correct in your assumptions.

Most vehicles have fairly high electrical draw just running the fans, perhaps around 200 W on high (measured the TSX at 220W). Even lower settings often have a high power consumption due to the resistors that many fans use to control speed. Factoring in the losses involved in generating that electricity, we might be looking at half a horsepower or more in required engine power.

A typical AC might take 3 HP when it runs, but since it runs intermittently (cycles on and off), the average power draw on the engine might be closer to 1.5 HP. The general rule is a 10% drop in MPG running the AC, though I suspect that also includes the power required to run the fans.

My best guess is that your fans drop MPG by about 4%, and if you were to run AC, it would drop it an additional 6%. If you have a gauge that measures fuel consumption at idle, you might be able to get a rough estimate of the extra amount of fuel needed to run the fans, as well as the extra amount to run the AC. You'd need to keep track of the % of time the AC is running vs % of time it is off to get an accurate average of the AC consumption. The other factor in AC power consumption is that it has to run a higher % of the time when cooling a heat-soaked car, and runs less frequently to maintain temperature in an already cooled cabin.

Cracking the windows is probably negligible depending on how open they are and the speed driven.