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Author Topic: Does driving a car with all the windows down consume more or less fuel?  (Read 4593 times)

Offline Chemistry4me

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Offline Geezer

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Erm, you left a few details out of your question, but, as summer is (supposed to be) approaching in the Antipods, we might assume you are referring to the vexing question of fuel cost associated with air conditioning in automobiles.

Apparently, assuming you have a reasonably modern vehicle, and you are travelling at typical highway speeds, you'll consume less fuel if you close the windows and turn on the AC than you would if you open the windows and turn off the AC. It's also likely that you will enjoy your journey a bit more with the windows closed.
 

Offline Chemistry4me

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My bad for not elaborating, let me re phrase the question: does driving a car with all the windows open consume more/less fuel than if all the windows were closed. But it seems you have partly answered it already Mr Geezer 
 

Offline Chemistry4me

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It's all about the aerodynamics then eh?
 

Offline CliffordK

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I don't have any vehicles with (working) AC... 

Nor do I have any vehicles with Real-Time MPG tracking.

When I was driving my Parent's Prius in the past and the AC was left on by someone else, I could see a difference of almost 5MPG on the real-time fuel efficiency reading, although perhaps I was inadvertently using both the windows down and the AC.  [xx(]

With my Ford Ranger, I tended to average a few MPG better fuel mileage in the summer with the windows down and one arm out the window than in the winter with the windows up.

Anyway, looking up on the WWW...  the conclusion I seem to get is that at low speeds (city driving), there is a moderate benefit from turning off the AC and driving with the windows down.

At highway speeds (60 MPH+), this difference may disappear, and the two may be equivalent.

http://www.primermagazine.com/2009/field-manual/know-it-all-what%E2%80%99s-better-for-fuel-economy-windows-rolled-down-or-air-conditioning

Here is a test by GM, I think showing that AC reduced fuel consumption about equally at all speeds, and worse for SUVs than for Sedans.  No Units on graphs.

http://www.sae.org/events/aars/presentations/2004-hill.pdf
 

Offline Chemistry4me

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Thank you CliffordK for the good answer. But I don't know where all the talk about A.C is coming from, allow me to state again the re phrased question: does driving a car with all the windows open consume more/less fuel than if all the windows were closed.
 

Offline Chemistry4me

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Sorry I missed this part ::)
With my Ford Ranger, I tended to average a few MPG better fuel mileage in the summer with the windows down and one arm out the window than in the winter with the windows up.
So it's windows down all the way then?
 

Offline peppercorn

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Sorry I missed this part ::)
With my Ford Ranger, I tended to average a few MPG better fuel mileage in the summer with the windows down and one arm out the window than in the winter with the windows up.
So it's windows down all the way then?

Clifford, I would suspect the slight fuel mileage improvement is because your engine prefers warm (and perhaps humid -?) air and that also offsets the imperceptible drop in MPG from having the window open.
[Unless you're not admitting to flapping an enormous Chinese fan with your free hand ;D]
 

Offline maffsolo

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Sticking to the airflow factors all heater / air conditioning airflow venting apparatus closed.
Having the weather being the same, external environmental wind flow is at zero.
No other factors to interfer.
Can the passenger compartment act as a parachute?

I believe the internal pressure of the compartment will act as a bubble. The pressure on the windshield great will deflect the wind and reduce the side low inward pressure. As long as the compartment bubble stays in tact the turbulance will glide by the sides of the vehicle, making no difference in foreward drag.

Now put the top down you may have a gas milage increase   
« Last Edit: 13/12/2010 11:55:19 by maffsolo »
 

Offline CliffordK

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The summer/winter difference is most certainly due to the heat.  And perhaps humidity as I had noticed that in the Midwest.  Some places also sell different fuel mixtures depending on the season.

The "bubble" theory is interesting.  And one might find a difference between the number and location of windows that are down, so merely having a drivers window down, and all other windows up might not be a problem.

This sounds like the pickup bed and tailgate argument with some research indicating that the mileage is worse with the tailgate down than it is with the tailgate up.

The PDF I posted earlier seemed to indicate a decrease in fuel efficiency with the windows down vs windows up and AC off.

I.E.

Best Mileage: Windows Up, AC Off
Medium: Windows Down, AC Off
Worst Mileage: Windows Up, AC On

Unfortunately they did not provide any units with their graphs, but otherwise the study seemed to be a well designed study.

I've driven without AC in 100 degree weather (Body Temp for those that use other units).  It is HOT with the windows down...  but unbearable when I must roll up the windows to talk on a cell phone.
 

Offline Geezer

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Leaving the AC bit out of the picture, unless you are travelling very slowly, the turbulence created by the open windows makes the engine work harder, and the faster you are travelling, the greater the effect becomes.

There is a similar question about driving with the tailgate on a truck up or down. A lot of people seem to think it saves fuel to drive with it down, but I believe there is some evidence that it's actually better to drive with it up, although it does seem a bit counterintuitive.
 

Offline peppercorn

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There is a similar question about driving with the tailgate on a truck up or down. A lot of people seem to think it saves fuel to drive with it down, but I believe there is some evidence that it's actually better to drive with it up, although it does seem a bit counterintuitive.

I saw a suggestion similar to this on a eco-modding site.  Something to with causing a vortex behind the closed tailgate that causes a spoiler for the air just above and coming off the back of the cab. .... Or something :-\
 

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