TomT wrote:It really makes no difference. To maintain a given temperature takes a given amount of energy. You will use the same amount of energy overall, either way. You are simply changing the energy loading from front to middle... Even with a given temperature set, you can still override the fan speed and still effectively be in auto by just manually changing the speed. The fan pulls very little, by the way, so it will have no practical difference on the range.
While that should be the case, try it and you'll find that - probably due to some minor flaws in software- it is not. When the temp is not set to 60*, the vehicle draws significantly more power at any given time. On the other hand, set it to manual, 60*, and use the fan to regulate temp - you'll notice immediately that it's drawing less power than (for example) setting it to 74* auto, and then you'll find that you end up in low within a few minutes even in very high outside temp/humidity, and once you reach that point climate control power draw will be next to nothing. For that matter until it gets into the upper 90's, I don't bother ever turning the fan up; even on low it cools the interior down fairly effectively in a short period of time.
I don't believe it is the fan that makes the difference; I believe it's two factors-
1) I believe that in auto mode the vehicle uses the heater to a small extent to make the air a specific temperature (ie 70*). This wastes energy in two ways; one, by utilizing the heater, and two, by requiring more cooling effort from the a/c system.
2) the compressor is utilized only as much as needed to maintain an evaporator core temp just above freezing. When the fan is low, the core absorbs less heat from the air, and as a result the compressor is hardly utilized to maintain that temperature. When the fan is high, the compressor is utilized more.
In short, there's a lot of room for software improvement in the way auto mode operates, but in the meantime you can get a much more efficient a/c just by keeping the temp on manual 60 and using the fan to control how much air you use. Before arguing that it shouldn't work, or that you don't think it would be effective, give it a try, you'll be surprised at how little power it uses, and how effective it is at cooling the cabin.
The one down side is that in milder temps the fan just doesn't go low enough... when temps are up to high 80's I find myself having to turn the climate control on and off to regulate temps... Even on the lowest fan setting it will get overly chilly at those temps. Would be nice to have another step in the fan controls that's about 1/2 of the current 'low' setting.
Still, is it more efficient than windows down? Who knows - at high speeds I'd bet on it. At low speeds, I'd bet it's below the point where most people in real roads with real day-to-day variations in conditions would be able to accurately measure the impact of either one.