AC or windows down?

http://ctwatchdog.com/2011/09/03/drivenct-which-uses-more-energy-car-ac-or-open-windows-you-might-be-surprized
DrivenCt: Which Uses More Energy - Car AC Or Open Windows? You Might Be
Surprized
By Hoffman Auto Group | Last updated Sep 9, 2011, 8:02 am
Written By Ron Winter
For decades automobile owners have debated whether air conditioning impedes gas mileage to a degree that it shouldn't be used, or whether the open-window "flow through" method of cooling is just as bad.
"It's a question we get a lot from people, especially during times of extreme heat," said Shawn Pinto, service manager for Hoffman Nissan. Pinto has been with Hoffman for over 10 years, and has experience with American, Japanese and German vehicles. "Drivers are always looking for ways to boost gas mileage and save money. Air conditioning use versus windows down is one of the longest-running debates for automotive experts."
Shawn Pinto
The argument goes that the air conditioner puts more strain on the engine because of parasitic draw when the air conditioning compressor is engaged, thus requiring extra gas to keep the vehicle at a constant speed.
Auto air conditioning proponents have long countered, however, that when a vehicle's windows are open the reverse pressure encountered when the air stream runs inside the car and up against the rear window also cuts down on the miles traveled per gallon to a degree that is comparable to using the air conditioner.
Recently a limited test on certain automobiles concluded that due to advance aerodynamic designs, running the vehicle with the windows open results in a negative impact that far exceeds the drain on the engine when the air conditioner is used.
The Society of Automotive Engineers tested both a full-sized sedan and a sport utility vehicle on an open track and in a wind tunnel, and as reported by HowStuffWorks.com, concluded "driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive."
SAE reported that "in the wind tunnel, air was forced over the front of the car and also from an angle on the front of the car to simulate a cross wind. In the desert, temperatures and vehicle speed were factored into the study. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency - more than using the vehicle's air conditioner."
For the sedan, when the windows were down, the efficiency was reduced by 20 percent, while the SUV fuel efficiency was reduced just 8 percent. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.
Since only two cars were studied under fairly limited driving conditions the results can hardly be considered absolutely conclusive. Nonetheless, the study does show that there is a significant disparity in fuel usage between a sedan and a SUV when comparing air conditioning and open windows - but in favor of using the air.
"Air conditioning versus windows down is a hard question to answer because there are so many variables that need to be accounted for," said Pinto. He explained that speed, weather conditions and the type of vehicle are all factors that contribute to the fuel efficiency of either AC or natural air. "The short answer to the debate is that maximum fuel efficiency, while staying cool, depends on the situation."
There are myriad arguments against using air conditioning, with some drivers preferring to purchase much smaller cars and hybrids - for their improved aerodynamics over SUVs and trucks - but often the issue goes much further.
According to the SAE study, driving at speeds in excess of 55 mph with the windows down result in a dramatic decrease in fuel efficiency - as much as 20 percent, versus around 10 percent with air conditioning. For this reason, many people refuse to drive over the old 55 mile per hour speed limit, even if they are in areas with 65, 70 or 75 mph speed limits. There are also some who drive only a night, or in the early morning and early evening hours to take advantage of cooler temperatures.
But aside from the speed issue, it is likely to be difficult for most people to alter their driving habits to such an extreme, in the interest of a potential savings of a few gallons of gasoline. And while driving a smaller vehicle or a hybrid certainly is a commendable effort, there is no way that approach will work for families with children.
Driving a long distance with an infant or toddler along for the ride can be difficult enough without also applying the extra burden of extreme discomfort. And there are reasons why some drivers prefer trucks, SUVs and full-sized sedans, including all the gear that growing families need when they take part in school sporting activities, summer vacations including camping, or carrying tools to the job site.
But if achieving maximum fuel efficiency - while staying comfortable - is situational, in what a setting do you choose to blast the AC?
"Highway driving is the most fuel-efficient use of air conditioning," said Pinto. "When you're driving around town, and having the windows down keeps you sufficiently cool, then don't use the AC."
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On Sun, 11 Sep 2011 19:09:11 -0400, "Ed Pawlowski"

Interesting article, but I might trust the Mythbusters more. What is "windows down?" I hardly ever do that any more if they mean all windows down.. Once in a while in city driving. Just did a 1500 mile trip in moderate temps without turning on the air once. Vent fan, and opened one window 1/2 inch. If that's not enough I turn on the A/C. Driving at 75 mph with a window open is plain uncomfortable. Didn't bother me when I was young though. Back in the day wing vents worked well too.
--Vic
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Maybe four times a year I'll drive around town with the windows down. Otherwise, like you, I find the noise is bothersome. I don't care about getting 1 mpg more if I need AC, it goes on. My last few cars had climate control. Set for 70 in the winter, 72 in the summer.
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Why?
Unless they revisited this one the episode I remember involved SUVs with fuel flow meters added and a track that was too small for them to go over 35MPH.
SUV=large space to condition, hence more A/C drag, less aerodynamic, hence less effect from windows open. This raises the balance point, AKA the speed at which open windows are better than A/C. Since they went 35 they were well under the balance point.
A ford Focus @ 70MPH would have had dramatically different results, let alone something with an electric A/C compressor like the '04-current Prius
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On Sun, 11 Sep 2011 22:37:50 -0500, "Daniel who wants to know"

I wasn't clear with my sarcasm. I don't trust the Mythbusters for any real science. The duct tape boat? I believed that. How high an exploding water heater can fly? I believed that. But the cited article was paid for by a Nissan dealership. The Nissan service manager quoted was pretty smart. "Air conditioning versus windows down is a hard question to answer because there are so many variables that need to be accounted for," said Pinto. He explained that speed, weather conditions and the type of vehicle are all factors that contribute to the fuel efficiency of either AC or natural air. "The short answer to the debate is that maximum fuel efficiency, while staying cool, depends on the situation."
They should have left it at that. Any SAE testing the writer mentioned was vague in detail. As I said before, what the does "windows down" mean? There are many "windows down" configurations, and they would give widely varied results. I drove a lot of cars with no A/C and don't remember ever having all windows wide open at highway speed except in a convertible. Without A/C the way to keep cool isn't to open all the windows. If you're alone you want air directed over you, not tearing out the headliner and buffeting your ears with pressure bouncing off the back window. Usually opening the drivers part way and the right rear an inch works well. With passengers in back they'll complain about the front windows being wide open and onrushing air squeezing their eyeballs. Anyway, BTDT.

I never saw that, but that shows the usual quality of Mythbuster "science."

Well, just shows some of the variance not covered in the article. It's all silly without stated test parameters - like what does "open windows" mean. This is the craziest thing in that article: "For this reason, many people refuse to drive over the old 55 mile per hour speed limit, even if they are in areas with 65, 70 or 75 mph speed limits." Just a few days ago I drove 1500 miles doing 2-10 over the limits of 65 and 70. Fairly heavy traffic. I didn't pass anybody except trucks going up steep grades. So I don't know where these "many people" doing 55 are. I sure didn't see any, and haven't for years.
--Vic
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Last time I drove a convertible, it was a rental. In the heat of the day, had the top down, windows down and the AC blowing on us. Last convertible I owned was in the early 1970's. It was a '64 Karmann Ghia that I paid $15 for. It was fun for a couple of years.
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Ed wrote: 'Last time I drove a convertible, it was a rental. In the heat of the day, had the top down, windows down and the AC blowing on us. Last convertible I owned was in the early 1970's. It was a '64 Karmann Ghia that I paid $15 for. It was fun for a couple of years." ****************************** I've had convertibles all my life and will never be without one. They're so much better than a motorcycle, of which I've had a few also. I get such a kick out of all the motorcycle riders thinking they're so cool, with their Harley-snob gear -- like they're really living "the life" and we're all supposed to be envious of them. Yeah, I really envy them while they're riding down the highway in the rain or cold, there's no way they aren't miserable. Even if it LOOKS like it might rain later, it casts a pall over the whole trip. Meanwhile, with a convertible, you still get ALL the sun and wind in your hair -- but you never get cold, never get wet, it's quieter, more comfortable, it's safer, WAY easier and thus less fatiguing, etc. You can take a drink or a smoke while you're driving, you can actually BUY something to take home with you that's larger than a pack of smokes, etc. etc. etc. And when the weather turns bad, you can simply put the top up. With a motorcycle you can't do ANY of that stuff -- all you can do is just drive it. Period. Carefully. Plus, you can't enjoy the scenery nearly as well, since you have to really concentrate as you're driving a bike -- and as such, after long enough -- it just turns into work... Whereas in a convertible you can spend the whole day comfortable and enjoying the sights. Motorcycles are the most over-rated thing in the world. To anyone who wants to ride them, I say knock yourself out, and be safe. But don't act like we all are supposed to envy you. Besides, all you did was walk into a bike shop and say, "Yeah, I'll take the red one.." Yet they wear that stupid Harley sweatshirt with more pride than someone in a Harvard shirt who graduated from there earning a Master's.
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wrote:

I enjoy watching the Mythbusters, but I beleive the Mythbuster's test of this was totally bogus. They drove too slow for aerodynamics to be significant. They drove SUVs which are so fuel inefficient that the A/C is such a minor power drain compared to the overall power required that it is difficult to tease out the A/C component. Also, didn't they deliberately set the AC to the coldest position, which would force the compressor to run more than would be the case with people just trying to stay comfortable?
I keep a log book on gas mileage. I've gone back and compared fuel economy versus month of the year for lots of venicles. I live in a realtively hot ara (North Carolina). You would think if A/C use was a major factor in fuel economy, then my fuel economy would be significantly worse in July and August than in April or October. It isn't. In fact, the I get some of my best mileage during the sumemr. For some vehicles I get my worst mileage during the deep winter months (December and Janruary), but I blame that on the winter fuel mixture. However I was surprised that I didn't usually get better mileage in spring and early fall months, since they have the summer gasoline type and realtively cool weather. I have gotten similar results with all sorts of vehicles (F150, Fusion, Expedition). I never drive with the windows down. However, I have had a couple of convertibles, and in the summer I often drove those with the top down and the A/C on at the same time. You would think that would really kill the fuel economy...but it didn't. Hardly any difference between summer and fall for those also.
I am now driving a Fiesta, and since it has a relatively small engine, is realtively areodynamic, AND has lots of glass area (making the car hotter in summer), I am thinking the A/C may actually reduce my summer mileage, but it is too early to tell.
I think for most people and most vehicles, the A/C is not a significant enough factor affecting fuel economy to be worried about. One more thing, just becasue the A/C is "on," it doesn't mean the compressor is actually on and consumiong power - it only runs enough to keep the evaporator in a certain tempearature range. Ed

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Of course, there is no free lunch, it has to take some small amount of power to run the AC, but I don't see it in my mileage either. There is enough variation tank to tank due to driving conditions that the AC is not a visible factor.
You mention that the compressor is not always running. Going back many years, I could tell when the AC kicked in because it took enough power that it could be felt while driving. Compressors have become more efficient and going back with my last four cars, I cannot tell you when the compressor kicked in at all. Yes, they were all V-6, but the power drain does not put enough load to make a real difference.
The last small car I drove was a Smart ForFour in Italy and I could tell on that, but we used the AC only a few times since it was cool. I'm going back in a couple of weeks and will again have a small car and the weather will be warmer. I expect on a smaller engine I may feel the power drain. We'll see.
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