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This is an age old debate that i have with a coterie of friends; without backing up with much scientific fact. It's all recently been based on arguments of experience.

Is it possible that utilizing the air conditioning in the car has a greater impact on fuel consumption/efficiency over having the window rolled down in a car?

chrisjlee
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    Which make and model and year of car are you wondering about? Each vehicle is designed differently (and are efficient in different ways), which I suspect may be a significant factor. – Randolf Richardson Aug 09 '11 at 01:59
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    @ Randolph, Older cars in the 70's used about 15 horsepower to rotate a compressor at full head pressure, modern ones are down to 3-5hp. So yes, it depends on a few factors. – Moab Aug 25 '11 at 01:17
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    Would have to depend on the speed of the vehicle, since wind resistance does not increase in a linear fashion, right? – PoloHoleSet Dec 19 '17 at 19:12
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    It would probably be dependant on the car's speed, as the A/C energy consumption would be fairly constant, whereas the drag induced by the open window would increase with speed (I'm not sure what the factor is but I think off the top of my head drag normally scales at the square of the speed) – GordonM Dec 20 '19 at 15:02

3 Answers3

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Mythbusters 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 traveling 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.

In comments Lagerbaer suggested that this

Makes sense from a physical point of view: A/C fuel consumption impact should be independent of actual velocity, whereas the drag effect of an open window will grow with the velocity (friction forces in general scale at least linearly with velocity)

While Michael Edenfield suggested the excellent point that

Since the physics concepts involved are the same for all cars, I suspect that more rigorous experiments would produce a different "cut-off point" than 50mph, but the answer to the OP's question is still "AC is better over x mph, worse under x mph". –

Also nic noted that 50 mph is approximately 80 km/h for those who use sensible units for speed.†

† I'm from the U.K. where we use m & mph for road signs and odometers, but metric for pretty much everything else. *8')

Community
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Mark Booth
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  • Makes sense from a physical point of view: A/C fuel consumption impact should be independent of actual velocity, whereas the drag effect of an open window will grow with the velocity (friction forces in general scale *at least* linearly with velocity). – Lagerbaer Aug 13 '11 at 21:56
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    I would note that this is not exactly a rigorous experiment. A high-quality experiment would require many car models with many different AC settings, and combinations of open windows. – user664939 Aug 15 '11 at 16:35
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    @user664939 - Agreed, and if you know of a rigorous, comprehensive, scientific study, please feel free to post an answer here and I will happily up-vote it. *8') – Mark Booth Aug 15 '11 at 16:47
  • @Mark Booth: Unfortunately, I don't :) . Mythbusters may be the best we've got. – user664939 Aug 15 '11 at 19:50
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    @user664939 Since the physics concepts involved are the same for all cars, I suspect that more rigorous experiments would produce a different "cut-off point" than 50mph, but the answer to the OP's question is still "AC is better over *x* mph, worse under *x* mph". – KutuluMike Aug 08 '12 at 14:58
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    50 mph = 80 km/h – nic Jun 04 '15 at 04:51
  • How about using the car's fan, without active cooling? If having the window down is sufficient for making it comfortable (i.e. it's not so hot outside that active cooling is required), the air circulation system might work too. – Sebastian Redl Dec 20 '17 at 09:28
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Edmunds.com conducted a test using a 2007 Toyota Tundra. Edmunds

They found that at 65MPH, fuel mileage was 19.1MPG with the a/c on, 20.9MPG with the windows down and air off.

Based on personal experience, I'd say that's pretty close to what you could expect across the board. Different makes/models will differ aerodynamically, but a 1-2MPG increase seems reasonable. Modern a/c compressors are somewhat more efficient than they used to be.

Whether that makes it worth it is up to the vehicle driver/owner. An increase of 2MPG at $2.50/gallon using the above example translates to savings of about $1.12 per 100 miles driven.

zlinedavid89
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    One must also consider whether the interior of the car is bearable at that speed with the windows down. This will depend on the temperature. – phoog Dec 14 '19 at 17:48
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    A Toyota Tundra is a truck with very poor aerodynamics. Thus lowering the windows doesn't change that much. Most modern cars are significantly better then pickup trucks so I'd expect the effect of open windows to be much greater. – Eric S Dec 23 '19 at 23:44
  • Aerodynamics would impact the overall mileage, but what this test was designed to do was gauge the change between windows open or closed. A vehicle could have great aerodynamic properties, but opening the windows are going to create additional drag regardless of how aerodynamic the vehicle is. There will be variances between different makes and models, but the question is still whether the additional engine load from the a/c is offset or exceeded by the aerodynamic drag from the open windows. – zlinedavid89 Jan 06 '20 at 16:23
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Mythbusters episode 22 tried this one, and they claimed it was busted.

geoffc
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    Well, what they found was that at high speed, AC is more efficient, but at low speed, opening the windows is. Makes sense, because air resistance increases rapidly with speed. Anyway, +1. –  Aug 09 '11 at 02:30
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    @geoffc - Can you provide details of the linked episode. We want people to be able to read the answers here in case referenced sites are taken down. – going Aug 09 '11 at 02:49
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    I'm trying to find the information again, but a few months ago I ran across what appeared to be a pretty good study on it. The results were similar in that windows open was best at low speed and A/C best at faster speeds. However, I seem to recall that the breakpoints were pretty high compared to what one would expect (75-90mph depending on the vehicle, and they had tested several). – Brian Knoblauch Aug 09 '11 at 18:19
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    Can you find a better source than mythbusters? – Sklivvz Jun 16 '16 at 06:40