If you notice I did put the "in many cases" cluase in my original statement. I'd trust my prof when it has something to do with drag. Fluids is this guys entire life(to the point I feel sorry for him). Since we also have an auto guy backing him up it sounds pretty sure.
Keep in mind drag is realated to surface area. By opening up a window you greatly increase the surface area of the body. Your also generating alot of turbulence around the openings. So a car with its windows down generates much more drag than with the windows up. Most likely a truck wouldn't feel the gain much becuase its drag is already very high because of the flat end of the cab and the sides of the bed... A small car is pretty aerodynamic and would take a huge hit from the windows being open. A modern AC unit in good condition is pretty efficent. Also if you really got into the math the added mass of the AC unit would reduce the acceleration due to drag on the car.
Either way I'm sure the difference between open windows and AC is minimal. You'd probobly have to due some controlled tests in a lab in order to see the difference.
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