I've heard that both are a drain on MPGs. It's more obvious with A/C use especially at higher temps. But I wonder if there is a point (a certain temp range) where the A/C running is actually better than having windows down?
But during my morning commute when it's only in the high 70's/low 80's I can drive to work with windows down (AC off) and get in the 60's mpg. In the evening when it's in the 90's and the A/C is working hard, i'm lucky to break 50 mpgs. So I'm not so sure it's an exclusive thing but more of a curve where at some point the curves cross.
Mythbusters did a story on this question and found that having the windows down was more fuel efficient than using the AC: Myth: Science/Transportation: AC vs Windows : MythBusters Granted, they used SUVs and not Prii, but the difference was fairly significant.
There is also the side benefit of A/C keeping the hybrid battery cool - when it gets too hot it's not as efficient/helpful as when it's cooler.
In low speed situations windows down for sure. In high speed situations 2 windows cracked 4" is best (flow through). On long trips then AC is ok and definitely more comfortable. You only take the big hit for a little while then it settles to about a 2 mpg hit which isn't bad. But it is still more of a hit then the flow through method.
I used my Air for the very first time today. The temps. here were in the 90's with very high humidity here. I was amazed at how much of a drain using it is on the mpg's. I was at just under 58mpg when I left the house and at 50.4 when I got home. Short stops are normally tough on mpg's but this was an eye opener. I will probably go with the windows down for the little short trips in the future. I think it will be better when I get back to commuting in a couple of weeks.........I hope.
Try the flow through method, djlen. Just crack your driver side window about 4-5" and do the same for your rear passenger side window. It will create air flow through the cabin. It can still get pretty wam so only use this method if you can remain cool enough to be relatively comfortable. You don't want to over heat your self or the HV battery.
When you must run the AC, try setting it just a couple of degrees below the ambient exterior temp shown on the display, with it set on recirculate of course. That way you will get the dehumidification effect but without the heavy load of a lot of cooling. When I do this it doesn't seem to affect mpg noticeably. At lower speeds (<40mph) I use the window method as described by F8L.
I'd have tried that today, but it was saying 99F when I got in the car and it only goes up to 85F. Generally I don't use A/C but when it's high 80s to 90s and opening the windows just makes the car feel hotter I stick it on 72 and curse. It was hot enough today that Auto's icy blast with high fan actually felt good. (I'm from the United Kingdom of Temperate so I like 68F and normally start whining when it gets to 75F )
At speeds under 35 mph, the fuel cost/impact of open windows on the Prius' fuel efficiency should be very small even when compared to a Prius with the AC off and the windows closed..
Agreed. The impact is probably small on most cars when going less than 35, I would think. They tested at 45 MPH.
When it's broiling hot or icicles hanging from the nose weather, I use the HVAC. A few MPGs isn't worth being in pain over!
I have found that it's A/C system is very efficient. In my Passat, I would have to turn on m A/C to a level 3 (out of 4). In my Prius, 90% of the time it is on the lowest bar. Rarely do I use over two bars and this is in the Panhandle of Florida (hell on Earth in the summer). I have not noticed any difference in MPGs.
Try ruining around without the AC for 7 tanks then turn it back on for 7 tanks then report back. We've tested it and the mpg loss is quite large in very hot conditions. Starting off from a cool garage and driving in 70-80F is not considered very hot so the hit will be noticeably lower. That is the only way I could see your AC system never going above two bars or you are never using Auto AC.
It's an aerodynamic issue. At lower speeds aerodynamics are not an issue, so windows-down wins. At higher speeds aerodynamics trumps cooling costs, so keep your windows up. The actual cutoff speed will depend on cooling load, but generally it will be somewhere around 50 mph. As for suggestions to crack the windows and such, yes, that works, but it doesn't provide a lot of cooling. Windows up and no AC produces even better mileage, but who wants to cook and bake their HV battery at the same time? This isn't an apples to apples comparison, but instead a subjective comparison. How much airflow equals how much cooling? The answer varies by condition and individual. Regarding Mythbusters, it's important to realize that they are not scientists or engineers. Many of their methods are highly suspect. It's an entertaining show, but don't quote it as gospel. Tom
Any on here with SGII, take the time to monitor your HV battery temps while trying to make these decisions also. I have been monitoring the middle battery temp since it is the highest, the battery fan inlet temp, the battery fan speed mode and the cabin temp. It has been in the mid-80's here but starting to climb in to the 90's. In the mornings I have been able to get by without the AC but in the afternoons after sitting in an open asphalt parking lot I have had to run the AC. If I do not run the AC in afternoons, even with highway driving, the battery temps continue to rise. With the AC set a 72 it will maintain battery temps and eventually begin to lower the temp as the cabin cools down. When I get off the highway and get in to start/stop driving the battery temps rise rapidly even with the AC on and the cabin temps in the 70's.
I can tell a lot of you are talking about extremes. But my initial post was more about the gray areas. Temps not too high, but when you are still looking for some cooling effects. Basically I was thinking there is a point where light AC use may actually be better than windows down. And more comfortable.
Dennis, I'm guessing in that "gray area" it's a lot less of a difference MPG wise and goes down more to comfort level. If the A/C is going to run using AUTO at 1 - 2 bars, the hard work for the A/C is done and it's just maintaining the temperature. This is typically when my MPG goes back up, even in around town driving.
Perhaps no more reputable around here than Mythbusters, but Consumer Reports had this to say: Fuel economy: Save money on gas Of course, the A/C and aerodynamic efficiency may vary, especially in a car like Prius.
