aldive said:
Your theory is sound. The ability to measure minute changes in gas mileage would require some elaborate testing equipment. In all of my testing by the usual method of calculating gas mileage, I have not observed any difference. I have made many long 300-500 mile highway runs testing this.
Have you actually tested this?
I thought about this more and re-read my original post - my post was intended to support your findings (to some extent), but may not have been as clear as I originally thought so let me try again...
The the 3 possibilities being discussed
(1) windows up w/out AC
(2) windows down w/out AC
(3) windows up with AC
Option (1) will always be the most efficient because drag is minimized and the compressor is not drawing any energy from the the engine. I don't think anyone is arguing this point.
On to the bone of contention: which is more efficient (2) or (3)? It depends on speed. Remember, drag is exponentially increased as speed increases so the faster you go the more drag you experience and the more engine work you lose to drag. Opening windows increases drag - I hope everyone can agree on this so I won't go into it. Suffice to say that the increase in noise when the windows are down is an indicator that you are losing energy to drag.
Unlike drag, the energy required by the compressor is (for all intents) constant, unaffected by speed. At some speed, the energy required to run the compressor is the same as the energy lost by the increased drag of the open windows (remember drag is increasing and compressor energy is staying the same). At this speed, neither open windows nor AC are any more efficient than the other. As speed increases beyond this point, drag causes more wasted energy than the compressor and the
AC (3) becomes more efficient. At what speed this situation occurs is completely dependant on the car, and will change from car to car - ie, there is less drag on a Formula 1 race car at 70 mph than there is on the Explorer (pretty close to the drag characteristics of a 4x8 sheet of plywood)
. That being said, Al may have found a speed (70 mph) at which the compressor "drag" is less than or equal to the window "drag." To compare his results to those of a '93 Explorer are like comparing apples to oranges - different cars, different drag coefficient. If one is waxed, that too, affects the drag and would yield different results.
I'm very surprised at the conclusions that have been drawn by some of these high profile tests (specifically, Consumer Reports and Mythbusters). I like both of them, so I was really surprised that neither of them made this distinction and generalized their findings across all cars at all speeds.
Their findings will only pertain to the car they tested, at the speed they tested. I hope this explanation makes that apparent. If one knew the actual energy required to run the compressor (can be calculated, knowing the efficiency of the compressor, etc) and the drag of the particular vehicle, one could determine the speed at which the two are equal.
Hope this makes sense. Great discussion.