I realize this has been discussed many times, but I thought I'd throw my own results in there. This is my evening commute, the same thing every time. Speeds of 30-45 mph. I put the A/C at about 75-76 when I use it. When I open the windows, I open the driver's window about 1/4 to 1/2 the way, and the back right window (the kitty-corner one) about 1/4 of the way. I find that gives good airflow and no low-frequency 'beat' (the back window cannot be lower than the front window). If you are going faster than 45 mph, or put all the windows all the way down, things may change, but for me it was pretty clear that using windows gives a lot better gas mileage than using A/C.
Yeah, I've noticed this too. Only, I keep all the windows down on a 10min commute 30-45mph fairly flat. I see close to 5-10mpg more that way. I can usually get the meter to stay above 50mpg after I get to the speed limits.
I usually don't notice A/C affecting mileage unless I end up sitting stopped somewhere, as it'll run down the mileage. I set to usually 72, recirculate, and fan speed on the second pip from the left (second slowest setting). But, I haven't tested and recorded data. It would interest me to see data for higher speeds. I know there is supposed to be a crossover point, and I've wondered where it is for the Prius.
I bet 45 for my '07 is that point. That's about where it gets pretty noisy in the cabin with windows down. Not a Dalek
I think that second test is a red herring, they didn't get the same amount of gas in, something like that. Plus, the way they keep referring to the AC as "blasting": it is possible to run the AC in auto mode with a higher temp set, and then it's impact likely is less. My take, if your're aiming for most efficiency: just use it at higher speeds, where air drag is more of a factor, and set the target ambient temperature around 75~76 F.
I'll add my 2 cents: the well-constructed WeatherTech side window deflectors for the front windows, with their front & top angled shield shape, do seriously modify outside airflow as it enters the cabin with the classic solution of a front + back pair of windows open. The low frequency cabin "beat" is eliminated at pretty much any speed. It seems the front air bubble of the moving car is extended outward enough to also decrease drag of open windows. In a year+ of using these deflectors during our hot & humid mid-Atlantic Summers, I found I need A/C rarely, as opposed to occasionally before the WeatherTechs. Frankly, unless its' cold or rainy I now close the window pair only when I'm on the phone Bluetooth while moving above 30+ MPH, to cut down on wind noise into the mike. I got these WeatherTechs to help ventilate the PiP when its' parked in the sun (everyday at work). Now I sense, without hard data, that the front deflectors likely also help mileage by decreasing A/C use and by creating a noticeably better airflow around the front window opening when the window pair is open. The back window deflectors don't stay in because of that window's opening shape - a top-tapering trapezium.
Just remember that heat is the enemy of HV battery life. Not using the AC in warm weather will decrease thy life expectancy of the HV battery and you will not be able to reuse most of your battery modules in a DIY rebuild as they will all have little capacity left. YMMV. JeffD
