Cars: Is it cheaper to :

[#1]

With the warm weather coming.

1. Drive with the windows down
2. windows up and air con on

(ignoring the obvious 3rd option of stripping off in the car, or sweating it out)

Having for many many years not having aircon I have been used to doing 1. But some people seem to claim it is actually cheaper using 2.
Not sure if it is only on longer runs (motorway speeds etc) or why.

Comments please and real life results would be great.

Of course when I am on the motorbike the air provides the cooling, but it is really hot when stopped. (In leathers/protective clothing)

[#2]

Unfortunately I don't have any personal experience from this, however if/when it actually warms up I could try and remember to take note trying each method. However I saw something on youtube a few days ago that was testing exactly this.

I'm pretty sure the results pointed out that option 2 works out cheaper on the motorway (As you've said) due to the drag caused by having the windows open at speed.

As a personal opinion not taking cost in to account much, I prefer having the aircon on when it does get really warm, or at least getting the car nice and cool with the aircon and then switch to having the windows open once it's cooled down a bit.

[#3]

When working in Nigeria I used to drive between towns with windows partially open air-conditioning off but in town windows shut with the air-conditioning on. On a long journey I experienced one third of the tank of petrol would be used to run the air-conditioning. I used to have a Peugeot 504 around 1980 and used to travel with two full jerry cans in the boot as there weren't many petrol stations 

[#4]

On a hot summer day I get in the car and put the air con on which cools the car nicely

All the passengers get in and immediately open their windows defeating the object of the air con

[#5]

http://auto.howstuffworks.com/fuel-efficiency/hybrid-technology/driving-with-windows-down.htm

Not surprisingly it depends on:-

Vehicle aerodynamics

Speed

and probably also whether the a/c is simple or climate control ( dosn't use compressor continuously )

I guess a typical car at motorway speeds with climate control is more fuel effiicent with windows closed

[#6]

Windows up, A/C on, especially as windows down means wasps get in and cause crashes...

[#7]

This theory of foreign objects in the car with the windows open was put to me by a very experienced IAM observer when I started doing my "lessons". I said in 30 years of driving I have NEVER had anything come in the car when it was moving, different matter when it was stopped.

Not sure what the AC is on mine, Seat Leon 54plate. The temp control is in degrees so maybe climate control?

I read that online article but remain unconvinced hence asking for real life experience vs a random web page.

[#8]

Pretty much as others have said, on a motorway driving with windows open causes a lot of drag (and noise) so generally better with A/C on and windows closed, at lower speeds (I generally recon <40mph as a vague guide) you can open the windows a bit (or a lot) and turn off the A/C.

[#9]

The lack of hot days make the question irrelevant at the moment.

[#10]

With my previous Focus, and present Fiesta, I've found negligible difference in fuel economy with AC on, so I always use it in preference to opening windows - plus I don't like the noise.

But, as @Strat says, passengers will open windows anyway, and then don't close them afterwards, which is most annoying to discover the following rainy morning! 

[#11]

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198kHz wrote:
passengers will open windows anyway, and then don't close them afterwards

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Can't think of a better reason to use the button that locks them and makes it so only the controls on the drivers door work

[#12]

You obviously have a posher car than me, @HarryB 

[#13]

I was watching a Mythbusters just the other day when they were testing this.  They were revisiting it from an earlier episode and came up with the same conclusions as have been said.

I tend to mostly use the climate control in mine though.

Phil

[#14]

Interesting Mythbusters there. Just watched the video.
Identical suv's on the track @ 45mph, the one with the windows down went 15 miles further than the ac one, 15% difference.
Looks like I have been doing it right all along. Will use the ac to cool the car if it is cooked when left but otherwise windows.

[#15]

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HairyMcbiker wrote:
Will use the ac to cool the car if it is cooked when left but otherwise windows.

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Although I do that myself, and I haven't tried this particular method, apparently opening the passenger window and then opening/closing the driver side door a few times is supposed to be good for initially cooling a car down as it forces all the warm air out of the 1 open window.