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Looking at gas milage and air conditioner use, it seems to me I get a significantly better gas milage when I don't use the air conditioner. However, if I roll down the windows, I assume this increases aerodynamic drag.

Under what circumstances does it make sense gas consumption wise to use an air conditioner vs rolling down the windows and vice versa if I want to reduce my gas consumption? For example would it make more sense to run the air conditioner when driving on the highway?

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    The thing I would like to know is what is the effect on vehicle drag depending on how far the window/s is/are open? Surely a window wound down completely creates significantly more drag than a window down 1cm or even 5cm. Most sites simply talk about "windows down" without stating how far they were open. Does anyone know of any research that looks at opening windows at speed vs aircon, when windows are open only 1-2 inches? – user1322 Apr 6 '14 at 11:42
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It's a more complicated answer that just saying "use your air con if you're going over X km/h".

When it comes to keeping cool in your car, there are other factors that make a big difference:

  1. On a cold but sunny day, the inside of a car can still get very hot, but opening a window just a little bit can reduce the temp to a comfortable level and is unlikely to substantially increase drag, no matter what your speed
  2. On a very humid hot day, opening windows all the way might not cool you down much, despite increasing drag
  3. The fan speed with which you run your aircon can affect how much power it draws, if I understand correctly how it works
  4. I'd guess that newer vehicles are more likely to have more efficient aircon that older vehicles, though it likely differs by model
  5. You may also be able to change the clothes you're wearing to something more comfortable

By the time you consider all those factors, you'd just about need to read a book on the subject to make the most efficient choice!

A sensible strategy to use would be this, in order:

  1. Choose comfortable clothing
  2. If the air temperature is reasonably cool, and you're not following a truck belching out stinky fumes, open windows the minimum amount necessary for comfort
  3. If you're still too hot, run the aircon, but reduce the fan speed (if it's manually controlled) to the minimum necessary level for comfort.

Also note the following: In my experience (and this will differ by manufacturer and vehicle age) the temperature setting in most vehicles shows a range from blue to red but the actual function of this setting is to provide a range of heating from none to lots. Don't run your aircon with the temperature setting half way between none and lots, because you're telling the car to heat the air again after you cool it! The one exception to this is when you want to defog your windscreen, where cooling then heating helps dry the air and thus defog more effectively.

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    I have to disagree with the first (1.) - the greenhouse effect can certainly heat up a car in the sun, even on a cold day, but once the car is moving, convective cooling is huge. You shouldn't need the windows open at all to cool the car, especially if the vents are set to Fresh air, not Recirculate. Regarding (3.), yes, that's how it works, but not because the fan uses much electrical energy. It's because you're increasing the Air Exchange Ratio, and forcing a new mass of warm air to be cooled more often. If the vents are on Recirculate, then this issue goes away ... – Nate Jun 8 '13 at 2:31
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    ... but then, inside air quality actually can suffer and be less healthy for occupants. Regarding the last point, it's true that medium settings (on older cars with internal combustion engines) do add heat to cooling. However, in an ICE, the heat is almost free, as it's waste from the engine. So, it's closer to correct to say that you don't save energy by keeping a cool vs. cold setting. On newer cars, though, they have thermostats and temperature setpoints, and can switch the compressor off. So, medium does save energy. – Nate Jun 8 '13 at 2:43
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As Highly Irregular suggests this very much depends on the specifics of your vehicle, conditions outside of your car and your desired conditions inside your vehicle.

When this question was brought up on skeptics the best evidence we could find was from Mythbusters which visited this topic in both episode 22 and episode 38:

Episide 22

PARTLY CONFIRMED *

Tests were performed under varying conditions (55 mph versus 45 mph). The 55 mph test used a computer to estimate fuel efficiency based on air intake, not actual fuel consumption, and showed A/C was more efficient. The 45 mph test consisted of running the tank until it was empty, and showed open windows were more efficient.

  • Because the original tests were inconclusive, this "urban puzzle" was revisited in episode 38: It is more fuel efficient to use air conditioning when the car is travelling approximately 50mph or more. Otherwise, windows are more fuel efficient.

Episode 38

PARTLY CONFIRMED

The fundamental flaw in the MythBusters’ test was that the point where the drag becomes powerful enough to inhibit a car’s performance with windows down was inside their 45 – 55mph margin at 50mph. Going less than 50mph it is more efficient to leave your windows down, but going greater than 50mph it is more efficient to use your A/C.

Sadly unless you have the same vehicle, are driving in the same conditions and have the same comfort requirements, this tells you little about your own situation.

I would suggest that the proof of the pudding is in the eating.

Next time you fill up, decide on a strategy and stick to it until your next fill up, then calculate your overall fuel efficiency. Then try a different strategy and see if the efficiency changes. From a series of such tests you can build up a body of empirical evidence about how your own vehicle, preferences and environment interact, and thus find out what is most sustainable for you.

  • Good answer, although sadly it does largely come to "it depends" ;-) It also depends a lot, I suspect, on the age of the car and the control system for the a/c. Some basic systems have an "A/C" button which, when pressed, has the compressor running pretty much full-time. In contrast, a modern "climate-control" type system will usually only run the compressor as much as necessary to maintain the set temperature (sometimes one must activate an "ECO" mode for this). – Flyto Apr 7 '14 at 9:15
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It was already answered in answers for a more general question about driving (it's just on the top in the linked answer): it depends on speed. The slowing effect of an open window depends on speed qudratically, but it becomes really significant only at high speeds (more than 100 km/h according to what I know, though I can try to back it by some source if you want).

I guess that air conditioning becomes clearly more gas-efficient than open window only on speeds forbidden on any highway except for those in Germany (no speed limit). That's why I never regret my car has no air conditioning :-)

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The amount of fuel consumption increased by using the AC in a car is not worth to consider. It only maters that it loses about 2 - 3 miles per galon. Is this worth to consider? If that bothers anyone, then why having a car in the first place? It involves much more things to worry then if the AC uses more petrol / diesel. The way you drive it make it a greater impact in the consumption then if the AC is on or off. Try to use 4th or 5th gear in town and 3rd when turning around, and you soon will see how much effective the consumption will be. I drive a 2.3 Seat Toledo (Same engine as in the MK 4 Golf V5) all year long with the automatic AC on at 21 C and with most the time using the higher gears in town and get around 35 mpg and mid 40 on a long run. This is a 11 years old 5 cylinder engine, which is less fuel effective then later engines. I do never drive with open windows anywhere, as this is the better way against of getting robed for the keys and car when stationary on red lights in town.

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    I found when I stopped driving with the AC on here in Jakarta, I started getting about 20-30% better fuel consumption. – Chris Travers May 28 '13 at 14:55
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    I think that trying to save fuel is worth considering. This site is about using natural resources without depleting them. Fossil fuels will be depleted some time, because we keep using them, they are a limited resource and they don't regenerate as fast as we use them. Still, considering alternative fuels (e.g. ethanol), it makes sense to figure out how to use as little as possible. The point of view you take in your answer is precisely the point of view that does not fit well with this site. But, please do stick around and make additions. Just don't be surprised if you receive down votes. – Earthliŋ May 28 '13 at 21:40
  • Even if it only loses 2-3mpg (which is probably reasonable for a modern climate-control system) that's up to a 5-10% impact on consumption, depending on the car. Seems significant to me. Of course other factors such as driving style can have greater effects, but there's no reason that one can't improve both. – Flyto Apr 8 '14 at 7:56

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