Hot window breeze or cool car breeze?

July 14, 2006 10:51 AM

What is more fuel efficient? Driving with your window(s) down, or with the air conditioner on?

I'm curious about which is more economical. I guess this would have to be answered in three parts:

1. Freeway driving only
2. City driving only
3. Overall driving (mix city/freeway)

I know that using the air conditioner in a car uses more gasoline, but having a window, or multiple windows, down creates more drag on the car. So which option is more fuel efficient?

Please note: I'm also not concerned about air quality. I realize that using the a/c in traffic is better as most cars have air filters to filter the smog. My question is basically focused on fuel consumption.

posted by chrisroberts to travel & transportation (23 comments total)

mythbusters
posted by blind.wombat at 10:53 AM on July 14, 2006

Mythbusters Dealt with this.

Air conditioning on is better.
posted by bitdamaged at 10:53 AM on July 14, 2006

doh!
posted by bitdamaged at 10:54 AM on July 14, 2006

Straight Dope.
posted by deadfather at 10:54 AM on July 14, 2006

There was a Mythbusters episode about this; someone grab asavage and let him answer.

ah, here we go: http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html

"Computer-based mpg measurements:

* 11.7/11.8 with A/C on and windows up
* 11.7/11.8 with A/C off and windows up
* 11.3 with A/C off and windows down

So, according to the computer, it's better to use A/C with windows up.

This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).

Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down."
posted by heeeraldo at 10:54 AM on July 14, 2006

Car Talk says using the air conditioner is generally more efficient.
posted by Wolfdog at 10:55 AM on July 14, 2006

that was the "revisited" one, I should add.
posted by heeeraldo at 10:55 AM on July 14, 2006

I just watched an installment about this exact thing on The Weather Channel this morning, and their conclusion (after running some tests) was that air conditioning was more fuel efficient than driving with the windows down, once you reached a certain speed, which I believe was 30 mph.
posted by iconomy at 10:56 AM on July 14, 2006

Mythbusters covered this, but I disagreed with their testing method, as it didn't conclude anything about the variable wind environments, variable car sizes, and driving environments.

I can only add my anecdotes: In my various cars over the years, I've found that underpowered cars tend to do better with windows down, rather than relying on Air Conditioning. In cars with a lot of horsepower, I've found that it doesn't seem to make a difference.

My wife's big fat Buick sedan works just as well, either way. It's a 250hp monster. The car I last had was a Kia Rio, which barely pegged over 100hp. I drove that death-box from Stamford, CT to Madison, WI (about 1200 miles) in the middle of August heat. I tried the AC at varying points in my drive, and each time, I could actually see the gas tank meter move down as I drove on the highway.

Add in the fact that the car didn't have cruise control, and that drive will probably live with me for a while...
posted by thanotopsis at 11:00 AM on July 14, 2006

You could get the worst/best of both worlds by simply turning on the fan. Very little gas consumption, no drag. Of course, in extreme heat, this won't help much. /me did this when driving 2 hours or so on less than a 1/4 tank of gas in a compact (and made it there with gas to spare, too!) [note to self: Next time remember to leave time to fill the car up...]
posted by shepd at 11:28 AM on July 14, 2006

The air conditioner is better for your health, not only because of the filters, but because the air is drawn from the high pressure area at the base of the windshield, which has fewer particulates. So if you consider your lifetime earning power, fewer sick days, and so on, A/C wins.
Not to mention that one bee coming in the window can run you off the road.
posted by weapons-grade pandemonium at 11:29 AM on July 14, 2006

I remember hearing something somewhere (maybe Cartalk) that it also depends upon the automobile. I seems like they said the A/C had little to no negative effect on fuel efficiency for something like a V-8 and negative effect for smaller 4 cylinders.
posted by Carbolic at 11:52 AM on July 14, 2006

To add a couple of other factors to this conversation, that will make it yet less certain of resulting in a definitive answer to the OP's question:

Most auto air-conditioners have a "max" cool setting, which is really a means of closing baffles in the air intake for the ventilation system, so that the air in the car is re-circulated through the air-conditioner, with little or no outside air being taken in. The result of this is faster cool down and greater dehumidification of the interior air, since recirculating the interior air a few times gets it very cold and dry. This takes less cooling energy by far than taking in 100+ cubic feet of air per minute, cooling it, and then losing it to vent leakage to the outside air (since the cooled air goes out of the passenger compartment quickly via pressure vent flaps when the system is set for fresh air intake). If you are not a smoker, and don't have a car full of people, running with the recirculating "max" setting may save some energy.

Next point is that automotive air conditioning systems vary significantly in the sophistication of the controls used. Ideally, a car's air conditioning system should be able to maintain a set point of temperature, by cycling the compressor on and off, according to the amount of heat energy that needs to be removed from the car to maintain that temperature. The only time the car's gas mileage would be negatively affected would be when the compressor's clutch is engaged by the control system, pulling energy from the engine to run the compressor. But some systems don't do this very well, and the compressor tends to run all or most of the time the air-conditioner is on, while the coolant is valved by solenoids. So, the design and sophistication of the air-conditioning system has a lot to do with the overall efficiency of it. Some cars will show very little effect of having the air-conditioner engaged because it is efficiently designed and well controlled, and others will run the compressor most of the time, because it is easier and cheaper to do. Unfortunately, cars advertised as high mileage vehicles often have poor air-conditioning systems, because EPA tests don't call for running the air-conditioning when making up fuel efficiency figures.
posted by paulsc at 11:52 AM on July 14, 2006

I also remember hearing something somewhere (maybe Cartalk) that it also depends upon the automobile. I seems like they said the A/C had little to no negative effect on fuel efficiency for something like a V-8 and negative effect for smaller 4 cylinders. I have a V-8 with about 300 hp and the A/C seems to have zero effect on gas mileage.
posted by Carbolic at 11:55 AM on July 14, 2006

To give some insight into why AC may be more efficient at high speeds:
-AC uses the same amount of power, no matter what speed you travel at
-At high speeds, drag is proportional to the square of speed:
Drag = const * s^2
and the power needed to overcome drag is speed times the force:
Power = const * s^3

Of course, you always have drag, but lowering the windows increases the drag (makes the constant coefficient bigger). So if speed is high enough, the additional contribution to drag will be greater than the power drawn by AC. However, since different cars have different aerodynamics, the speed where AC becomes more efficient will vary from car to car.

Also, because drag grows like the cube of speed, countering it is the main source of fuel consumption at highway speeds. If you're considering making yourself uncomfortable (choosing a cooling option that you don't otherwise prefer) just to increase fuel efficiency, why not just drop your speed by 5 mph instead. Better yet, use cruise control ---driving 70 mph is more fuel efficient than spending half your time at 65 mph and half at 75 mph.
posted by Humanzee at 11:56 AM on July 14, 2006

I also remember having an issue with the mythbuster's methodology, in particular the almost-certainly-error-inducing fuel removal situation. I personally do not understand why they didn't buy some fluid flow sensors and get a perfect measurement of what was coming out of the tank...

Every indication is that modern cars are very carefully tuned for aerodynamics (with the beetle being an glaring exception) and having the windows open notably alters that performance. The compressor being on may impact total power but not to the extent that wind drag can.

WGP, I don't know you can make that blanket a statement considering all the icky crawlies that have been shown to grow in AC ductwork given the right conditions...
posted by phearlez at 11:58 AM on July 14, 2006

This is really going to depend. On cars where the air flow is well attached to the sides of the car at speed opening the windows creates turbulence. If the flow is already turbulent then the effect of opening the windows is minimalized. Also the horsepower needed to operate the compressor and the condenser fans varies greatly from vehicle to vehicle and over all efficiency is highly dependant on ambient temperatures. The faster you go the more important the added drag if any.

phearlez writes "I personally do not understand why they didn't buy some fluid flow sensors and get a perfect measurement of what was coming out of the tank..."

Because they are quacks, they couldn't get a room full of gasoline to catch fire for example, and their methods are sloppy. Their measurements errors are probably lost in the noise of the variables of vehicle type/cd and environmental factors. Something as simple as a 1lb difference in tire pressures between the two vehicles could account for the difference.

phearlez writes "WGP, I don't know you can make that blanket a statement considering all the icky crawlies that have been shown to grow in AC ductwork given the right conditions..."

Ya, you won't catch legionnaire's from fresh air. And even the recirc setting (which not all cars have) on a auto A/C isn't air tight
posted by Mitheral at 12:18 PM on July 14, 2006

There was a recent study that examined this issue in detail. Can't find a link at the moment, sorry, but the gist of it was that although it varies from car to car, for most sedans the speed at which it becomes more efficient to keep close windows and use the A/C is about 80 km/h (50 mph). So a perhaps a rule of thumb might be to use the A/C only when you're going at steady highway speeds, and roll down the windows at any other time.
posted by randomstriker at 12:56 PM on July 14, 2006

Consumer Reports tests the mileage of a lot of cars. They recently published a statement that they have found the overall decrease in mileage from using AC clearly exceeds that from opening the windows. That's on the entire large sample of cars they test, at the wide range of speeds they run them at.

Your car, or your speed might give a different result, but if you want one answer, that's it.
posted by Kirth Gerson at 1:17 PM on July 14, 2006

The original study claiming that A/C was more efficient was done years ago by DuPont ... who just happened to produce and sell Freon®. Doesn't mean it wasn't true, of course.

Last November, edmunds.com tested six fuel economy tips to see if they saved any gas. They couldn't measure a practical difference with A/C vs. windows, so they concluded that you might as well be comfortable and use A/C. It was an interesting article.
posted by pmurray63 at 3:29 PM on July 14, 2006

Another data point in an endless stream of speculation:

At idle, most air-conditioning control systems kick up the engine idle speed by 300 to 500 rpm, to overcome the additional load presented by the air-conditioning compressor, and keep the engine from dying while idling at stop lights. You can test this yourself on any car equipped with air-conditioning (easier if it also has a tachometer), by putting the car in neutral and turning the air-conditioner on and off. The difference in rpm you see on the tach, or the difference in engine sounds you hear, is the effect of the idle speed bump device kicking your idle speed up. So, if you are in city or stop and go traffic, using the air-conditioner will definitely cost you some additional energy vs. not using it.

The air-conditioner evaporator is also in front of the radiator on most vehicles, so running the air-conditioner also adds heat load to the engine cooling system at idle. If you are running your air-conditioning while idling on a hot day, you can see under hood temperatures as much as 15 to 30 degrees higher, shortening the life your engine's belts, oil, and rubber parts.
posted by paulsc at 7:10 PM on July 14, 2006

paulsc writes "The air-conditioner evaporator is also in front of the radiator on most vehicles"

The condensor is in front of the rad, the evaporator is the part that gets cold and is mounted inside the car.
posted by Mitheral at 8:30 PM on July 14, 2006

Here's a whole big statement from CR dealing with how to Get the Most Milage out of your car. Regarding A/C

USING AIR CONDITIONING VS. OPENING THE WINDOWS

Air conditioning places more load on the engine, which can affect fuel economy. But some auto journalists say that opening the windows at highway speeds can affect fuel economy even more by disrupting the vehicle's aerodynamics. In our tests we found that neither makes enough of a difference in fuel economy to worry about. Using air conditioning while driving at 65 mph reduced gas mileage in both vehicles by about 1 mpg-it might make more of a difference if you drive faster. The effect of opening the windows at 65 mph was not even measurable. Because air conditioning can help keep you comfortable and alert and because most modern cars use it to keep windows defrosted, we suggest that the small trade-off in fuel economy for increased safety is worthwhile.

They also list a whole bunch of things that DO effect milage, like driving styles (no sudden or quick acceleration please), so it's a good read.
posted by tiamat at 8:49 PM on July 14, 2006