Does using your car's air-conditioning use up gas faster?

Josh111485 · 24th September 2009, 12:41 PM

Does using your car's air-conditioning use up gas faster than if you didn't use it? Should we just roll down our windows in the summer time to save on gas?

jhunter1163 · 24th September 2009, 12:43 PM

Mythbusters covered this. IIRC, using the AC cuts fuel economy by about 10% compared to driving with the windows down.

SK. · 24th September 2009, 12:44 PM

Well rolling down the window increases the drag coefficient of the car, so the answer is not trivial.

Unlike a Bull · 24th September 2009, 12:54 PM

IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down.

shawmutt · 24th September 2009, 12:57 PM

http://en.wikipedia.org/wiki/MythBus...._Windows_Down

Quote:

Tests were performed under varying conditions (55 miles per hour (89 km/h) versus 45 miles per hour (72 km/h)). Also, the 55 mph test was using 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.

You should just buy a motorcycle, I get 60mpg on mine and don't have to fiddle with silly AC buttons.

McHrozni · 24th September 2009, 01:17 PM

Originally Posted by Unlike a Bull

IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down.

Of course, the difference in comfort between the two options is substantial.

McHrozni

ponderingturtle · 24th September 2009, 01:18 PM

Originally Posted by jhunter1163

Mythbusters covered this. IIRC, using the AC cuts fuel economy by about 10% compared to driving with the windows down.

Depending on speed. At high speeds windows down was less efficient.

casebro · 24th September 2009, 01:20 PM

Best possible gas mileage: roll up the windows, use your fan's 'vent' option . If it as one. My F-250 fan system does NOT intake cool outside air, it only recirculates. I think they wanted to sell buyers an add on AC system.

Just thinking · 24th September 2009, 01:21 PM

How about cars that put their tops down?

(Just make sure you wear a hat ... sunstroke is easy to get that way.)

BobTheDonkey · 24th September 2009, 01:27 PM

Originally Posted by ponderingturtle

Depending on speed. At high speeds windows down was less efficient.

Ahh, but they revisited the issue and found that they were incorrect about that.

The fuel mileage computer in the vehicle is based on air intake, not fuel usage. So it was not an accurate assessment. They, in fact, mention this at the beginning of their 2nd round of testing with limited fuel in each vehicle.

Ziggurat · 24th September 2009, 02:25 PM

Originally Posted by Unlike a Bull

IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down.

The details will vary. As a general rule, the energy cost of running AC is independent of velocity, whereas the increase in drag from open windows will depend upon velocity. So we should expect that with any car, there will be a crossover point in efficiency (windows being more efficient at low speeds, AC being better at high speeds). Where that crossover happens depends not only on the car you drive, it will also depend on how hard you run the AC (it will happen at a lower speed if the AC is on a lower setting). It is rare for most people to need to run the AC at close to full for any prolonged period, and such cases usually involve very hot weather where opening the windows wouldn't help much anyways.

ponderingturtle · 24th September 2009, 02:26 PM

Originally Posted by BobTheDonkey

Ahh, but they revisited the issue and found that they were incorrect about that.

The fuel mileage computer in the vehicle is based on air intake, not fuel usage. So it was not an accurate assessment. They, in fact, mention this at the beginning of their 2nd round of testing with limited fuel in each vehicle.

And they only tested it a 45 mph not 65 mph or 75 mph.

BobTheDonkey · 24th September 2009, 02:49 PM

Car and Driver did a test in 2007:

http://www.caranddriver.com/features...itioner_page_2

Their results were that driving at 75mph with the AC on caused a 6.8% loss in fuel mileage, while driving with the windows down at 75mph caused only a 2% loss in fuel mileage.

ETA: In all fairness, there are other studies that did not show a significant difference in fuel mileage between AC and windows down at approx 70mph. The key to all of this is the vehicle. Windows down on a full-size truck or SUV doesn't affect the aerodynamics of the brick as much as windows down on a modern sleek sedan. Ultimately, on the highway it's about which is more comfortable while around town is more efficient with the windows down.

jasonpatterson · 24th September 2009, 04:22 PM

There are other advantages and disadvantages beyond fuel economy. At 40mph in nice scenery and decent weather, having the windows down is just more pleasant, regardless of the fuel. If you're on the interstate, having the windows down creates a raging gale that prevents conversation and beats the snot out of you. Even if it is more fuel efficient, windows down isn't much of an option when you're going 80mph unless you're a diehard motorcyclist or convertible lover.

Windows have a relatively restricted range of usefulness compared to AC as well. That said, I am one of those guys with one arm on the window frame driving at 80mph in 50 degree F rain. Maybe I'm part golden retriever...

Jeff Corey · 24th September 2009, 07:31 PM

You stick your nose out the window? Here's one point of view, "The choice of car cooling technique is a tricky one. While the question of air conditioning (AC) versus rolled-down-windows has not yet been conclusively answered, we can be confident in a few things. First, air conditioning does dramatically reduce the fuel economy of your vehicle at any speed. Second, lowering your car windows also reduces fuel economy by increasing drag. This drag becomes worse as your speed increases. The question remains: at what speed does the drag overcome the gain from not using your air conditioning? A study done by the Society of Automotive Engineers found that cars were more efficient with the AC off and windows down for all speeds up to about 65 mph, but that the margin of difference narrowed at higher speeds. So, for now we recommend rolling the windows down below highway speed and using the air conditioning while on the highway."
I would guess that wouldn't work in Woolamaroo or Death Valley.

gnome · 24th September 2009, 07:36 PM

What about these ads I'm seeing for the Prius using solar-powered ventilation? Is it as brilliant in practice as it sounds in theory? Seems to mesh awesomely with the main weakness of solar--you need AC mainly when the sun is out--or at least more of it.

casebro · 24th September 2009, 07:59 PM

Originally Posted by BobTheDonkey

Car and Driver did a test in 2007:

http://www.caranddriver.com/features...itioner_page_2

Their results were that driving at 75mph with the AC on caused a 6.8% loss in fuel mileage, while driving with the windows down at 75mph caused only a 2% loss in fuel mileage.
.

Sooo, at 20 mpg, and $3.00 per gallon, the 5% difference is equal to .0075 cents per mile. sounds pretty moot.

Jeff Corey · 24th September 2009, 08:00 PM

Originally Posted by gnome

What about these ads I'm seeing for the Prius using solar-powered ventilation? Is it as brilliant in practice as it sounds in theory? Seems to mesh awesomely with the main weakness of solar--you need AC mainly when the sun is out--or at least more of it.

Look at this. Does it cool the car when you're on the road? http://jalopnik.com/5232393/2010-toy...ilation-system

arthwollipot · 24th September 2009, 08:09 PM

In my Series 1 Prius, it makes a substantial difference. The air conditioning only actually runs when the IC engine is running - it can't condition just on electric. When I'm stopped at the lights, the IC engine normally switches off. If I'm running the air conditioning, it doesn't switch off.

Originally Posted by McHrozni

Of course, the difference in comfort between the two options is substantial.

Absolutely. Especially when air conditioning is most required - when it's really hot. When it's so hot outside that the air coming in the window is hot, rolling down the window doesn't do much for cooling down the car.

BobTheDonkey · 24th September 2009, 08:09 PM

Originally Posted by casebro

Sooo, at 20 mpg, and $3.00 per gallon, the 5% difference is equal to .0075 cents per mile. sounds pretty moot.

Honestly, I don't use the AC sparingly to save money (for the same reason I won't drive across town just to save a few pennies on the gallon - it's not worth the hassle). I've found I can easily get 300 miles to the tank without the AC, something like 250-275 with. So, by running long road trips without the AC, I don't have to stop as often (or like a few weekends ago when my gf and I made a 250mi trip - keeping the AC off gave me enough extra fuel to just drive it straight through).

mhaze · 24th September 2009, 08:10 PM

Originally Posted by Josh111485

Does using your car's air-conditioning use up gas faster than if you didn't use it?

Rule 1: Car only runs if AC is on.

Originally Posted by Josh111485

Should we just roll down our windows in the summer time to save on gas?

By Rule 1, then the car would NOT run.

shawmutt · 24th September 2009, 08:56 PM

Originally Posted by jasonpatterson

There are other advantages and disadvantages beyond fuel economy. At 40mph in nice scenery and decent weather, having the windows down is just more pleasant, regardless of the fuel. If you're on the interstate, having the windows down creates a raging gale that prevents conversation and beats the snot out of you. Even if it is more fuel efficient, windows down isn't much of an option when you're going 80mph unless you're a diehard motorcyclist...

I usually roll up the windows on my bike when going highway speeds

casebro · 24th September 2009, 10:22 PM

I usually carry a car door on my bike. That way, when it gets hot, I can roll down thew window.

Cuddles · 25th September 2009, 07:03 AM

This isn't really an issue in the UK, since we don't actually have summer here.

Jeff Corey · 25th September 2009, 07:36 AM

Originally Posted by Cuddles

This isn't really an issue in the UK, since we don't actually have summer here.

Really?
"Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey."
http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm

paximperium · 25th September 2009, 08:05 AM

The AC leaches power from the engine at a preset rate, whether idling the car or driving at 80mph. An electric pump for the AC will help fix this problem since the power for the AC at that point will come from a battery but in general, any use of the AC leads to some fuel efficiency loss.

The issue with having your windows down is way more complicated due to plain old auerodynamic variation of each car. Some cars, especially convertibles, can drive at high speeds without any change in their drag coeffiecient since they were designed to so so or some have such terrible drag coefficient that is makes no differences(box vans). Some cars will suffer a massive change in their drag coefficient with their windows down(I believe the Prius is known for this).

A general rule of thumb is that it becomes more fuel efficient to keep your windows down and use your AC at highway speeds but not all cars follow this rule.

LTC8K6 · 25th September 2009, 08:07 AM

The 5.7L Hemi in my Jeep doesn't seem to care...

NoZed Avenger · 25th September 2009, 08:11 AM

Originally Posted by Jeff Corey

Really?
"Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey."
http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm

So he was correct. Not a real summer at all.

/Texas

paximperium · 25th September 2009, 08:19 AM

Originally Posted by LTC8K6

The 5.7L Hemi in my Jeep doesn't seem to care...

When your Jeep has a drag coefficient of 1, I don't believe the windows being down matters.

uruk · 25th September 2009, 09:04 AM

Originally Posted by Jeff Corey

Really?
"Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey."
http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm

Lucky Brits, It was @110F for nearly two months here in South Texas.

We finally recieved our first cold front of the season. It's a nice overcast 75F.

Ziggurat · 25th September 2009, 09:05 AM

Originally Posted by paximperium

The AC leaches power from the engine at a preset rate, whether idling the car or driving at 80mph. An electric pump for the AC will help fix this problem since the power for the AC at that point will come from a battery but in general, any use of the AC leads to some fuel efficiency loss.

An electric pump wouldn't necessarily help at all. If you're drawing current from the electrical system, then you're increasing the load on the alternator, so the engine still has to work harder. Now you might be able to do that more efficiently than with a direct mechanical connection between the engine and the pump, but you can't avoid increasing the load on the engine by running the AC.

paximperium · 25th September 2009, 09:11 AM

Originally Posted by Ziggurat

An electric pump wouldn't necessarily help at all. If you're drawing current from the electrical system, then you're increasing the load on the alternator, so the engine still has to work harder. Now you might be able to do that more efficiently than with a direct mechanical connection between the engine and the pump, but you can't avoid increasing the load on the engine by running the AC.

Agreed. I overstated it since I was thinking hybrid powertrains but electric AC systems will benefit normal IC cars as well. It will be more efficient and give more options in how the AC is run since it is no longer tied to the engine, it can vary the power load more efficiently.

ElMondoHummus · 25th September 2009, 09:18 AM

There are definitely times when it becomes necessary to run AC at low temperatures, and it has nothing to do with the weather being hot. The defroster/defogger by default uses air from the AC (or at least it does on most cars). It works better than putting it on "Economy" because untreated air from the outside may be too humid to effectively keep your window from fogging up, whereas air conditioning helps dehumidify the air it outputs and clears your window better.

So in spite of the reduction in fuel efficiency, there are honest reasons to run your AC at city driving speeds. On a day where the windows fog up at the drop of a hat, you actually need your AC to see outside the car.

LTC8K6 · 25th September 2009, 10:34 AM

Originally Posted by paximperium

When your Jeep has a drag coefficient of 1, I don't believe the windows being down matters.

Actually though, it runs on 4 cylinders in cruise and does quite well on fuel.

The Cd is .40, not 1.

LTC8K6 · 25th September 2009, 10:43 AM

My auto climate control does whatever it does anyway.

paximperium · 25th September 2009, 10:47 AM

Originally Posted by LTC8K6

My auto climate control does whatever it does anyway.

So that's what they call rolling your windows down nowadays?

LTC8K6 · 25th September 2009, 10:51 AM

Originally Posted by paximperium

So that's what they call rolling your windows down nowadays?

Ha!

I don't know but it's smart. If I remote start it when it's cold out, it automatically turns on the defrosters, including the electric ones for the back window and the mirrors.

Tormac · 25th September 2009, 11:17 AM

A corollary to Murphy’s Law dictates that what ever is the most effective at cooling you is also the most injurious to your gas millage.

For example while driving slowly in stop and go traffic one will not get much of a breeze with the windows rolled down, while having the air conditioning on will effectively cool down the car. Therefore widows down is more efficient, and the air conditioner hurts mileage more whiel driving in city traffic.

The opposite is true when traveling at seventy five miles an hour on the highway.

jasonpatterson · 25th September 2009, 01:00 PM

Originally Posted by casebro

Sooo, at 20 mpg, and $3.00 per gallon, the 5% difference is equal to .0075 cents per mile. sounds pretty moot.

A small correction: At 5% savings,$3.00 per gallon, and 20mpg, you're spending $0.15 per mile, 5% of that is $0.0075 or 0.75 cents. Still not a big deal, you'd have to travel almost the entire way across the US to save $20. I guess if you were consistent about it and drove your car an average amount, you'd save about $100 every 6 years or so at this rate.

RussDill · 25th September 2009, 05:30 PM

Don't forget that car windows are also safety devices.

24th September 2009, 12:41 PM	#1
Josh111485 Scholar Join Date: Oct 2008 Location: Los Angeles, CA Posts: 122	Does using your car's air-conditioning use up gas faster? Does using your car's air-conditioning use up gas faster than if you didn't use it? Should we just roll down our windows in the summer time to save on gas?

24th September 2009, 12:43 PM	#2
jhunter1163 Beer-Swilling Semiliterate Join Date: Jul 2006 Location: Room 118, Bohemian Grove Marriott Posts: 5,812	Mythbusters covered this. IIRC, using the AC cuts fuel economy by about 10% compared to driving with the windows down.
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24th September 2009, 12:44 PM	#3
SK. New Blood Join Date: Oct 2007 Posts: 24	Well rolling down the window increases the drag coefficient of the car, so the answer is not trivial.
SK. New Blood Join Date: Oct 2007 Posts: 24

24th September 2009, 12:54 PM	#4
Unlike a Bull Scholar Join Date: Sep 2008 Posts: 86	IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down.
Unlike a Bull Scholar Join Date: Sep 2008 Posts: 86

24th September 2009, 12:57 PM	#5
shawmutt Squirrel Murderer Join Date: Jan 2009 Location: Greencastle, PA Posts: 1,209	http://en.wikipedia.org/wiki/MythBus...._Windows_Down Quote: Tests were performed under varying conditions (55 miles per hour (89 km/h) versus 45 miles per hour (72 km/h)). Also, the 55 mph test was using 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. You should just buy a motorcycle, I get 60mpg on mine and don't have to fiddle with silly AC buttons.
	Last edited by shawmutt; 24th September 2009 at 12:59 PM.

24th September 2009, 01:17 PM	#6
McHrozni Muse Join Date: Jun 2009 Posts: 690	Originally Posted by Unlike a Bull IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down. Of course, the difference in comfort between the two options is substantial. McHrozni
McHrozni Muse Join Date: Jun 2009 Posts: 690

24th September 2009, 01:18 PM	#7
ponderingturtle Orthogonal Vector Join Date: Jul 2006 Location: Ossining NY Posts: 18,758	Originally Posted by jhunter1163 Mythbusters covered this. IIRC, using the AC cuts fuel economy by about 10% compared to driving with the windows down. Depending on speed. At high speeds windows down was less efficient.
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24th September 2009, 01:20 PM	#8
casebro Illuminator Join Date: Jun 2005 Posts: 3,727	Best possible gas mileage: roll up the windows, use your fan's 'vent' option . If it as one. My F-250 fan system does NOT intake cool outside air, it only recirculates. I think they wanted to sell buyers an add on AC system.
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24th September 2009, 01:21 PM	#9
Just thinking Illuminator Join Date: Jul 2004 Location: New Jersey Posts: 4,542	How about cars that put their tops down? (Just make sure you wear a hat ... sunstroke is easy to get that way.)
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24th September 2009, 01:27 PM	#10
BobTheDonkey Muse Join Date: Jun 2009 Posts: 911	Originally Posted by ponderingturtle Depending on speed. At high speeds windows down was less efficient. Ahh, but they revisited the issue and found that they were incorrect about that. The fuel mileage computer in the vehicle is based on air intake, not fuel usage. So it was not an accurate assessment. They, in fact, mention this at the beginning of their 2nd round of testing with limited fuel in each vehicle.
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24th September 2009, 02:25 PM	#11
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 13,871	Originally Posted by Unlike a Bull IIRC, mythbusters did a follow up to that one and found that as you go faster the drag from the open windows increases and it becomes more effecient to use your AC. I think that was at 55MPh though I suppose your particular car and your particular AC sytem would effect what that particular speed is for you. For normal city driving you're better off with your windows down. The details will vary. As a general rule, the energy cost of running AC is independent of velocity, whereas the increase in drag from open windows will depend upon velocity. So we should expect that with any car, there will be a crossover point in efficiency (windows being more efficient at low speeds, AC being better at high speeds). Where that crossover happens depends not only on the car you drive, it will also depend on how hard you run the AC (it will happen at a lower speed if the AC is on a lower setting). It is rare for most people to need to run the AC at close to full for any prolonged period, and such cases usually involve very hot weather where opening the windows wouldn't help much anyways.
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24th September 2009, 02:26 PM	#12
ponderingturtle Orthogonal Vector Join Date: Jul 2006 Location: Ossining NY Posts: 18,758	Originally Posted by BobTheDonkey Ahh, but they revisited the issue and found that they were incorrect about that. The fuel mileage computer in the vehicle is based on air intake, not fuel usage. So it was not an accurate assessment. They, in fact, mention this at the beginning of their 2nd round of testing with limited fuel in each vehicle. And they only tested it a 45 mph not 65 mph or 75 mph.
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24th September 2009, 02:49 PM	#13
BobTheDonkey Muse Join Date: Jun 2009 Posts: 911	Car and Driver did a test in 2007: http://www.caranddriver.com/features...itioner_page_2 Their results were that driving at 75mph with the AC on caused a 6.8% loss in fuel mileage, while driving with the windows down at 75mph caused only a 2% loss in fuel mileage. ETA: In all fairness, there are other studies that did not show a significant difference in fuel mileage between AC and windows down at approx 70mph. The key to all of this is the vehicle. Windows down on a full-size truck or SUV doesn't affect the aerodynamics of the brick as much as windows down on a modern sleek sedan. Ultimately, on the highway it's about which is more comfortable while around town is more efficient with the windows down.
BobTheDonkey Muse Join Date: Jun 2009 Posts: 911	__________________ [As] to the pointy of moral lessons, I may as well take them from Bruce Lee as Jesus. He's contradictory, fallible, arrogant...But at least I know he existed. - Red3 Last edited by BobTheDonkey; 24th September 2009 at 02:53 PM.

24th September 2009, 04:22 PM	#14
jasonpatterson Philanthropic Misanthrope Join Date: Apr 2009 Location: Macedonia, OH Posts: 903	There are other advantages and disadvantages beyond fuel economy. At 40mph in nice scenery and decent weather, having the windows down is just more pleasant, regardless of the fuel. If you're on the interstate, having the windows down creates a raging gale that prevents conversation and beats the snot out of you. Even if it is more fuel efficient, windows down isn't much of an option when you're going 80mph unless you're a diehard motorcyclist or convertible lover. Windows have a relatively restricted range of usefulness compared to AC as well. That said, I am one of those guys with one arm on the window frame driving at 80mph in 50 degree F rain. Maybe I'm part golden retriever...
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24th September 2009, 07:31 PM	#15
Jeff Corey New York Skeptic Join Date: Aug 2001 Posts: 9,731	You stick your nose out the window? Here's one point of view, "The choice of car cooling technique is a tricky one. While the question of air conditioning (AC) versus rolled-down-windows has not yet been conclusively answered, we can be confident in a few things. First, air conditioning does dramatically reduce the fuel economy of your vehicle at any speed. Second, lowering your car windows also reduces fuel economy by increasing drag. This drag becomes worse as your speed increases. The question remains: at what speed does the drag overcome the gain from not using your air conditioning? A study done by the Society of Automotive Engineers found that cars were more efficient with the AC off and windows down for all speeds up to about 65 mph, but that the margin of difference narrowed at higher speeds. So, for now we recommend rolling the windows down below highway speed and using the air conditioning while on the highway." I would guess that wouldn't work in Woolamaroo or Death Valley.

24th September 2009, 07:36 PM	#16
gnome Philosopher Join Date: Aug 2001 Posts: 6,412	What about these ads I'm seeing for the Prius using solar-powered ventilation? Is it as brilliant in practice as it sounds in theory? Seems to mesh awesomely with the main weakness of solar--you need AC mainly when the sun is out--or at least more of it.
gnome Philosopher Join Date: Aug 2001 Posts: 6,412	__________________

24th September 2009, 07:59 PM	#17
casebro Illuminator Join Date: Jun 2005 Posts: 3,727	Originally Posted by BobTheDonkey Car and Driver did a test in 2007: http://www.caranddriver.com/features...itioner_page_2 Their results were that driving at 75mph with the AC on caused a 6.8% loss in fuel mileage, while driving with the windows down at 75mph caused only a 2% loss in fuel mileage. . Sooo, at 20 mpg, and $3.00 per gallon, the 5% difference is equal to .0075 cents per mile. sounds pretty moot.
casebro Illuminator Join Date: Jun 2005 Posts: 3,727	__________________ Please pardon me for having ideas, not facts. Some have called me cynical, but I don't believe them. It's not how many breaths you take. It's how many times you have been breathless that counts.

24th September 2009, 08:00 PM	#18
Jeff Corey New York Skeptic Join Date: Aug 2001 Posts: 9,731	Originally Posted by gnome What about these ads I'm seeing for the Prius using solar-powered ventilation? Is it as brilliant in practice as it sounds in theory? Seems to mesh awesomely with the main weakness of solar--you need AC mainly when the sun is out--or at least more of it. Look at this. Does it cool the car when you're on the road? http://jalopnik.com/5232393/2010-toy...ilation-system

24th September 2009, 08:09 PM	#19
arthwollipot The Accidental Podcaster Join Date: Feb 2005 Location: On the other side of your screen. Posts: 28,268	In my Series 1 Prius, it makes a substantial difference. The air conditioning only actually runs when the IC engine is running - it can't condition just on electric. When I'm stopped at the lights, the IC engine normally switches off. If I'm running the air conditioning, it doesn't switch off. Originally Posted by McHrozni Of course, the difference in comfort between the two options is substantial. Absolutely. Especially when air conditioning is most required - when it's really hot. When it's so hot outside that the air coming in the window is hot, rolling down the window doesn't do much for cooling down the car.
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24th September 2009, 08:09 PM	#20
BobTheDonkey Muse Join Date: Jun 2009 Posts: 911	Originally Posted by casebro Sooo, at 20 mpg, and $3.00 per gallon, the 5% difference is equal to .0075 cents per mile. sounds pretty moot. Honestly, I don't use the AC sparingly to save money (for the same reason I won't drive across town just to save a few pennies on the gallon - it's not worth the hassle). I've found I can easily get 300 miles to the tank without the AC, something like 250-275 with. So, by running long road trips without the AC, I don't have to stop as often (or like a few weekends ago when my gf and I made a 250mi trip - keeping the AC off gave me enough extra fuel to just drive it straight through).
BobTheDonkey Muse Join Date: Jun 2009 Posts: 911	__________________ [As] to the pointy of moral lessons, I may as well take them from Bruce Lee as Jesus. He's contradictory, fallible, arrogant...But at least I know he existed. - Red3

24th September 2009, 08:10 PM	#21
mhaze Philosopher Join Date: Jan 2007 Location: Woo*(+-1.10)^20=AGWwoo Posts: 8,230	Originally Posted by Josh111485 Does using your car's air-conditioning use up gas faster than if you didn't use it? Rule 1: Car only runs if AC is on. Originally Posted by Josh111485 Should we just roll down our windows in the summer time to save on gas? By Rule 1, then the car would NOT run.
	__________________ Get your climate widget Climategate - History's Biggest Scam Poptech's List Climate Acronyms

24th September 2009, 08:56 PM	#22
shawmutt Squirrel Murderer Join Date: Jan 2009 Location: Greencastle, PA Posts: 1,209	Originally Posted by jasonpatterson There are other advantages and disadvantages beyond fuel economy. At 40mph in nice scenery and decent weather, having the windows down is just more pleasant, regardless of the fuel. If you're on the interstate, having the windows down creates a raging gale that prevents conversation and beats the snot out of you. Even if it is more fuel efficient, windows down isn't much of an option when you're going 80mph unless you're a diehard motorcyclist... I usually roll up the windows on my bike when going highway speeds

24th September 2009, 10:22 PM	#23
casebro Illuminator Join Date: Jun 2005 Posts: 3,727	I usually carry a car door on my bike. That way, when it gets hot, I can roll down thew window.
casebro Illuminator Join Date: Jun 2005 Posts: 3,727	__________________ Please pardon me for having ideas, not facts. Some have called me cynical, but I don't believe them. It's not how many breaths you take. It's how many times you have been breathless that counts.

25th September 2009, 07:03 AM	#24
Cuddles Decoy Moderator Join Date: Jul 2006 Location: A magical land full of pink fluffy sheeps and bunnies Posts: 10,317	This isn't really an issue in the UK, since we don't actually have summer here.
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25th September 2009, 07:36 AM	#25
Jeff Corey New York Skeptic Join Date: Aug 2001 Posts: 9,731	Originally Posted by Cuddles This isn't really an issue in the UK, since we don't actually have summer here. Really? "Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey." http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm

25th September 2009, 08:05 AM	#26
paximperium Philosopher Join Date: May 2008 Location: Silicon Valley-Stuck between Google and Apple Posts: 7,800	The AC leaches power from the engine at a preset rate, whether idling the car or driving at 80mph. An electric pump for the AC will help fix this problem since the power for the AC at that point will come from a battery but in general, any use of the AC leads to some fuel efficiency loss. The issue with having your windows down is way more complicated due to plain old auerodynamic variation of each car. Some cars, especially convertibles, can drive at high speeds without any change in their drag coeffiecient since they were designed to so so or some have such terrible drag coefficient that is makes no differences(box vans). Some cars will suffer a massive change in their drag coefficient with their windows down(I believe the Prius is known for this). A general rule of thumb is that it becomes more fuel efficient to keep your windows down and use your AC at highway speeds but not all cars follow this rule.
	__________________ "The method of science is tried and true. It is not perfect, it's just the best we have. And to abandon it with its skeptical protocols is the pathway to a dark age." -Carl Sagan "They say a little knowledge is a dangerous thing, but it's not one half so bad as a lot of ignorance."-Terry Pratchett

25th September 2009, 08:07 AM	#27
LTC8K6 Philosopher Join Date: Nov 2002 Location: Directly under a deadly chemtrail Posts: 6,967	The 5.7L Hemi in my Jeep doesn't seem to care...
	__________________ What a fool believes, no wise man has the power to reason away. What seems to be, is always better than nothing. 2 prints, same midtarsal crock..., I mean break?

25th September 2009, 08:11 AM	#28
NoZed Avenger Bitter Whiner Join Date: Apr 2002 Posts: 8,793	Originally Posted by Jeff Corey Really? "Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey." http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm So he was correct. Not a real summer at all. /Texas
	__________________ There is no substitute for good manners. Except fast reflexes.

25th September 2009, 08:19 AM	#29
paximperium Philosopher Join Date: May 2008 Location: Silicon Valley-Stuck between Google and Apple Posts: 7,800	Originally Posted by LTC8K6 The 5.7L Hemi in my Jeep doesn't seem to care... When your Jeep has a drag coefficient of 1, I don't believe the windows being down matters.
	__________________ "The method of science is tried and true. It is not perfect, it's just the best we have. And to abandon it with its skeptical protocols is the pathway to a dark age." -Carl Sagan "They say a little knowledge is a dangerous thing, but it's not one half so bad as a lot of ignorance."-Terry Pratchett

25th September 2009, 09:04 AM	#30
uruk Illuminator Join Date: Apr 2003 Location: In the land of the Shatner stealing Mexico touchers Posts: 4,380	Originally Posted by Jeff Corey Really? "Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey." http://news.bbc.co.uk/2/hi/uk_news/england/5193970.stm Lucky Brits, It was @110F for nearly two months here in South Texas. We finally recieved our first cold front of the season. It's a nice overcast 75F.
	__________________ Facts are meaningless, they can be used to prove anything that is even remotely true. Just because I don't care doesn't mean that I don't understand. Why do things that happen to stupid people keep happening to me!?! Lisa!...In this house we obey the laws of thermodynamics! Alot of great things are based on lies, like religion and American history Homer Simpson

25th September 2009, 09:05 AM	#31
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 13,871	Originally Posted by paximperium The AC leaches power from the engine at a preset rate, whether idling the car or driving at 80mph. An electric pump for the AC will help fix this problem since the power for the AC at that point will come from a battery but in general, any use of the AC leads to some fuel efficiency loss. An electric pump wouldn't necessarily help at all. If you're drawing current from the electrical system, then you're increasing the load on the alternator, so the engine still has to work harder. Now you might be able to do that more efficiently than with a direct mechanical connection between the engine and the pump, but you can't avoid increasing the load on the engine by running the AC.
	__________________ "There is certainly not one government in Europe but is now watching the war in this country, with the ardent prayer that the united States may be effectually split, crippled, and dismember'd by it... We are all too prone to wander from ourselves, to affect Europe, and watch her frowns and smiles. We need this hot lesson of general hatred, and henceforth must never forget it. Never again will we trust the moral sense nor abstract friendliness of a single government of the world." - Walt Whitman, 1864

25th September 2009, 09:11 AM	#32
paximperium Philosopher Join Date: May 2008 Location: Silicon Valley-Stuck between Google and Apple Posts: 7,800	Originally Posted by Ziggurat An electric pump wouldn't necessarily help at all. If you're drawing current from the electrical system, then you're increasing the load on the alternator, so the engine still has to work harder. Now you might be able to do that more efficiently than with a direct mechanical connection between the engine and the pump, but you can't avoid increasing the load on the engine by running the AC. Agreed. I overstated it since I was thinking hybrid powertrains but electric AC systems will benefit normal IC cars as well. It will be more efficient and give more options in how the AC is run since it is no longer tied to the engine, it can vary the power load more efficiently.
	__________________ "The method of science is tried and true. It is not perfect, it's just the best we have. And to abandon it with its skeptical protocols is the pathway to a dark age." -Carl Sagan "They say a little knowledge is a dangerous thing, but it's not one half so bad as a lot of ignorance."-Terry Pratchett

25th September 2009, 09:18 AM	#33
ElMondoHummus 0.25 short of being half-witted Join Date: Oct 2006 Location: Somewhere north of the South Pole Posts: 7,404	There are definitely times when it becomes necessary to run AC at low temperatures, and it has nothing to do with the weather being hot. The defroster/defogger by default uses air from the AC (or at least it does on most cars). It works better than putting it on "Economy" because untreated air from the outside may be too humid to effectively keep your window from fogging up, whereas air conditioning helps dehumidify the air it outputs and clears your window better. So in spite of the reduction in fuel efficiency, there are honest reasons to run your AC at city driving speeds. On a day where the windows fog up at the drop of a hat, you actually need your AC to see outside the car.
	__________________ How can a pickup truck contain enough mass to unfold into a towering machine? I say if Ringling Brothers can get 15 clowns into a Volkswagen, anything is possible. -Roger Ebert, Transformers review ElMondoHummus, you are a silly, silly man. -Hokulele

25th September 2009, 10:34 AM	#34
LTC8K6 Philosopher Join Date: Nov 2002 Location: Directly under a deadly chemtrail Posts: 6,967	Originally Posted by paximperium When your Jeep has a drag coefficient of 1, I don't believe the windows being down matters. Actually though, it runs on 4 cylinders in cruise and does quite well on fuel. The Cd is .40, not 1.
	__________________ What a fool believes, no wise man has the power to reason away. What seems to be, is always better than nothing. 2 prints, same midtarsal crock..., I mean break?

25th September 2009, 10:43 AM	#35
LTC8K6 Philosopher Join Date: Nov 2002 Location: Directly under a deadly chemtrail Posts: 6,967	My auto climate control does whatever it does anyway.
	__________________ What a fool believes, no wise man has the power to reason away. What seems to be, is always better than nothing. 2 prints, same midtarsal crock..., I mean break?