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

[Josh111485]

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]

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

[SK.]

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

[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.

[shawmutt]

http://en.wikipedia.org/wiki/MythBusters_(2004_season)#AC_vs._Windows_Down

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]

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]

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]

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]

How about cars that put their tops down?

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

[BobTheDonkey]

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]

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]

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]

Car and Driver did a test in 2007:

http://www.caranddriver.com/features/08q4/gas_pains_mileage_myths_and_misconceptions-feature/turn_off_your_air_conditioner_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]

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]

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]

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]

Car and Driver did a test in 2007:

http://www.caranddriver.com/features/08q4/gas_pains_mileage_myths_and_misconceptions-feature/turn_off_your_air_conditioner_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]

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-toyota-prius-solar-powered-ventilation-system

[arthwollipot]

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.

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]

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]

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.

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]

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 :D

[casebro]

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

[Cuddles]

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

[Jeff Corey]

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]

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]

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

[NoZed Avenger]

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]

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]

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]

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]

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]

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]

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. :D

[LTC8K6]

My auto climate control does whatever it does anyway. :D

[paximperium]

My auto climate control does whatever it does anyway. :D
So that's what they call rolling your windows down nowadays? :cool:

[LTC8K6]

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

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]

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]

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]

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

[BobTheDonkey]

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

Yup, that's why they must be rolled down when you take your car on the track...b/c they don't care about your safety there...

[bruto]

I think this is probably one of those problems that cannot be answered entirely in a general way, because it depends not only on the speed, but the vehicle, how the windows work, how much percentage of the car's horsepower is used for the AC, how air is directed, etc.

If an air conditioner uses a given horsepower, then from the percentage point of view it will always make more sense to use it on a big engine than a small one.

It depends a little on how many windows you open, and how much. Back in the days of openable wing windows, I would bet that windows would always beat AC at speed, because it's possible to scoop a lot of air through a wing window without upsetting the aerodynamics of the whole car.

I greatly dislike systems that use the AC for defrosting. While it's true that the AC will dehumidify and defog very quickly, it chills the window so that you must leave it on all the time, and even run the wipers on cold damp days. Air conditioned defogging ends up being a self-feeding addiction. Heat-only defrosting takes more time, but once it's done, the warm glass resists further fogging.

I should add that before AC became so common, I think cars had much more efficient and useful cold air systems. The cars I drove in the 60's and 70's would blow lots of cool air through their vents at highway speeds even without a fan. There's something to be said for simplicity.

[quarky]

At what temp does wind-chill stop being a 'factor'?

Rather a complex graph.

I like to drive naked. I have a bottle of alcohol rigged to the ceiling that spritzes me at certain temps for evaporative cooling. I try not to get any in my mouth. (Officer.)

[dafydd]

I don't own a car.

[CORed]

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.

Not correct. You will have losses in both the alternator and the motor. Auto AC's do not run the compressor continuously. They have an electromagnetic clutch which cycles the compressor on and off. Usually they are set to keep the evaporator above freezing, so it doesn't ice up, but an adjustable thermostat inside the passenger area certainly could be done, with either an electric or belt driven compressor.

Also, an electric compressor would require a higher capacity alternator and heavy guage wiring, or possibley a higher voltage electrical system.

[Tebtenri]

Given that for experimental purposes there is one person in the vehicle, AC would work quite efficiently at speeds of less than 100kph (60mph) but the drag on the vehicle increases giving a decrease in fuel economy at 100kph up to 25% less.
Though with more passengers the AC definitely is working hard to maintain a constant cooling, also vehicles made today are not vented as older vehicles were and are actually constructed for AC.
For demisting the AC it only needs to be on for a minute or so.

[arthwollipot]

Really?
"Wednesday was the hottest July day in England ever, with a temperature of 36.5C (97.3F) taken at Wisley, Surrey."Pffff!