Quote:
Originally Posted by terry1156
Here's my question: Do you use more fuel with the air conditioning on and windows rolled up or the air conditioning off and the windows rolled down? aIf id depends on speed, at what speed does the change take place?
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This was on an episode of Mythbuster. If you're pressed for a semi-scientific answer you may search for it.
Generally speaking, a vehicle with the windows rolled down creates more aerodynamic drag than one with a vehicle with the window roll up. The higher the speed of the vehicle = the higher the drag resistance.
We also know that the air conditioning compressor becomes more efficient with higher engine speed. The faster you go = the more negligible the a/c resistance becomes.
If we plotted the parasytic losses on a graph, the point where the two lines intercept would depend on the shape of the vehicle and the efficiency of the engine.
There really is no way to know exactly what that speed is because vehicles are different, but the general consensus in the industry is that at highway speeds, you're better off closing the windows and blasting the a/c.
Sorry but I don't think anyone can give you a better answer than that without putting your vehicle in a wind tunnel.