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Old 30th August 2010, 13:16   #1
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Default Windows down or A/C on -- which is more fuel-efficient?

Came across good article on HowStuffWorks , Link is

Howstuffworks "Windows down or A/C on -- which is more fuel-efficient?"

Windows down or A/C on -- which is more fuel-efficient?

If you're concerned about fuel economy, when (if ever) does it make sense to use your vehicle's air conditioner?

There are two main factors to consider when approaching this question. The first has to deal with how the air compressor in your car works and how much extra fuel the engine has to use to keep it running. The second is what is known as air resistance or drag. Drag is the resistance that cars, and all moving objects, encounter when moving through the air at any speed. Most modern cars are designed to be relatively aerodynamic, which allows them to pass through the air with minimal resistance.
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However, when a vehicle has its windows down, air passes into the car where it was formerly allowed to flow over it, causing resistance that didn't exist when the windows were up. You can think of it a bit like a parachute. When a skydiver opens up the parachute, it cups the air and causes a massive amount of drag, enough to slow the speed of the skydiver and allow him or her to the land safely on the ground. Unlike the parachute, you definitely don't want a lot of drag on your car because it makes your engine work harder to get your vehicle up to the same speed.

So, does drag really affects a car's fuel economy more than air conditioning?
When to Use Your Air Conditioner

It depends on your driving speed, but there are times when using the air conditioning system in your car can be more fuel efficient than rolling the windows down.Based on a study conducted by the Society of Automotive Engineers (SAE), driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive [source: Hill]. We'll get around to discussing when air conditioning isn't a more efficient option, but let's first take a look at what the SAE found.
The SAE study was conducted at a General Motors wind tunnel and on a desert track. In the wind tunnel, air was forced over the front of the car and also from an angle on the front of the car to simulate a cross wind. In the desert, temperatures and vehicle speed were factored into the study. Two vehicles were used in the test, one was a full size SUV with an 8.1-liter V-8 engine and the other was a full-size sedan equipped with a 4.6-liter V-8 engine. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency -- more than using the vehicle's air conditioner.
For the sedan, when the windows were down, the efficiency was reduced by 20 percent, while the SUV fuel efficiency was reduced just 8 percent [source: Hill]. These differences are an important factor in determining just how much the windows down option will affect the fuel efficiency of your vehicle. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.
When driving at speeds of more than 55 miles per hour (88.5 kilometers per hour) with the windows down, there's a decrease in fuel efficiency of 20 percent or more. Although using the air conditioner decreases fuel efficiency as well, cooling the air through the compressor only decreases the fuel efficiency by about 10 percent [source: Arthur].
So, when traveling at speeds around 50 miles per hour (80.5 kilometers per hour) or faster, air conditioning is usually a better bet, but what about when you're simply cruising around town? Keep reading to find out how you can save a little bit of fuel on those slow-speed short trips.
When to Roll the Windows Down

There are times when it's more fuel-efficient to drive with the windows down -- and the fresh air feels pretty good, too.When you're driving around town at relatively low speeds, you'll use less gas by switching the air conditioner off and rolling down the windows [source: Arthur]. It's more efficient to drive with the windows down at slow speeds as opposed to faster speeds because there's less aerodynamic drag when you're driving slower [source: Motavalli].
As your speed increases, however, the amount of drag on the vehicle will also increase. But the drag doesn't increase in a linear fashion, it increases exponentially. For example, when your vehicle is traveling at a speed of 70 miles per hour (112.7 kilometers per hour), there's actually four times more force on the vehicle than when you're cruising around at 35 miles per hour (56.3 kilometers per hour). So even though the vehicle's speed is doubled, the drag is actually increased by four times.
If you're searching for a good rule-of-thumb number for when it's best to open the windows and switch off the air conditioner, according to some experts, the cut-off should be around 40 miles per hour (64.4 kilometers per hour) [source: Arthur]. What's the reason? Well, at low speeds your engine is producing less power, so it would have to work much harder to power accessories like the air compressor. When the engine is operating at faster speeds, it's already producing ample power for both the engine and additional equipment [source: Austin].
Although we've made the case for both windows down and air conditioning, some argue that the windows down option is still the better bet. Car and Driver did its own study and determined that you should switch off your air conditioner -- most of the time [source: Austin]. However, the U.S. Department of Energy recommends using both windows down and air conditioning when the conditions permit [source:U.S. Department of Energy].
So, if you're conscious of going green to reduce fuel consumption, or if you're just wanting to save some green and make fewer stops at the pump, then combine both windows down and air conditioning use. If you're smart about when you choose either option, you'll save a little bit of gas and stay cool while doing it, too.
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Old 30th August 2010, 14:08   #2
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I would not go into technicalities, leave it to the experts to answer. All I would say is rolling the windows up and putting the air con ON gives me
1) No exterior noise
2) No pollution
3) Peacefully enjoy the music or a conversation.

If I am spending a little more for fuel to achieve this, I can live with it. The only time I switch off the Aircon is when I am at a signal about to move & roll down the windows during the summers when I have push the hot air outside to ensure faster cooling. I am happy spending slightly more on fuel to accomodate the air con operation.
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Old 30th August 2010, 14:15   #3
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Very simple. Windows open mean more turbulence and a not so clean profile. With speed sir resistance increases as the 4th power of the velocity. The moral

at low speeds the windows open, no AC configuration will be more efficient. At speed the AC+windows closed will be more efficient.
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Old 30th August 2010, 14:19   #4
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Quote:
Originally Posted by ghodlur View Post
I would not go into technicalities, leave it to the experts to answer. All I would say is rolling the windows up and putting the air con ON gives me
1) No exterior noise
2) No pollution
3) Peacefully enjoy the music or a conversation.

If I am spending a little more for fuel to achieve this, I can live with it. The only time I switch off the Aircon is when I am at a signal about to move & roll down the windows during the summers when I have push the hot air outside to ensure faster cooling. I am happy spending slightly more on fuel to accomodate the air con operation.
+1 to that, my thoughts similar.

I adhered this principle so much that my safari power windows have gone kaput (Got it rectified now anyways)
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Old 30th August 2010, 14:31   #5
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Another big advantage of keeping the windows rolled up and using the ac 100% is that the interiors remain clean forever. Moreover you feel fresh even when the weather outside is humid though not hot as is normally the case in coastal towns even in winters.

I drive with 100% ac on.

Murthy
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Old 30th August 2010, 14:37   #6
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I get best FE with windows down all the time.

The best i've achieved with A/C is 18.9kmpl while i've got 22.8kmpl without A/C at cruising speeds of 80-100kmph.

The power loss through turbulence at these speeds is nowhere as much as the load, put on the engine by the compressor.

Shan2nu
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Old 30th August 2010, 14:44   #7
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I actually thought about this quite a while ago and do the following to get the best of both whenever prossible -- I have my windows up and the AC off, and simply I set the blower at high speed.
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Old 30th August 2010, 18:16   #8
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I think not many of us know this facts.
I also came across such results first time and we are still thinking that AC
is a major culprit in mileage , this is very interesting finding
so rule of thumb is

When driving at speeds of more than 80-88 kilometers per hour with the windows down, there's a decrease in fuel efficiency of 20 percent or more, but cooling the air through the compressor only decreases the fuel efficiency by about 10 percent
So, when traveling at speeds around 80.5 kilometers per hour or faster, air conditioning is usually a better bet
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Old 30th August 2010, 19:33   #9
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Best FE on my Safari TCIC so far with AC-ON is 15kmpl. And I have never achieved that figure again with windows rolled down fully.
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Old 30th August 2010, 19:58   #10
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I too had heard and believed it quite a long time ago that modern day cars have powerful engines which do not strain the engine a lot for additional power to power the AC and it is actually more fuel efficient to roll up the windows with the AC on to reduce the coefficient of drag.
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Old 31st August 2010, 17:26   #11
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Within the city (< 60 - 80 kph), air-conditioner off. On the highway (over 80 kph), air-conditioner on. This will get you the best FE.

BUT there is more to life than an extra 1 - 2 kpl. If you own a car, you obviously care about convenience. Hence, let your comfort levels - and NOT FE - dictate whether you need the air-conditioner on.
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Old 31st August 2010, 17:54   #12
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AC on always. I simply hate dust. More so if it is inside the car. And i like good noise insulation, which is possible only when the windows are rolled up.

Adjust the thermostat and blower settings according to the outside temperature (in absence of an ACC) and you won't have that much of a difference in FE.

Last edited by DRIV3R : 31st August 2010 at 17:56.
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Old 31st August 2010, 19:42   #13
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Unless you are in a hill station in Chikmagalur (!), I suggest that you keep windows up, AC on, lowest fan speed in recycle mode. Your car Interiors stay clean and the time and amount that you saved to clean it can be used for your 1-2 kpl difference.
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Old 31st August 2010, 19:54   #14
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If you enjoy the air pollution, noise pollution and privacy invasion that comes from the extra mileage borne out of lowered windows, go for it !
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Old 31st August 2010, 20:10   #15
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100% A/C daytime.
On/Off at night (If I drive alone)

Interior is less dusty
In Chennai with the crazy temperature you got to be insane to run with A/C off during day. (Just my view)

Car is run with A/C off only on track. That too with windows rolled up 95% for obvious reasons.
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