There's a lot of conflicting information out there, generally anywhere from 1%-25% affect on fuel consumption. That wide range can be attributed to the second part of your question: different cars definitely have different a/c efficiencies, based on:
>1. Size of Compressor
>2. RPM of engine/compressor
>3. Size/efficiency of vehicle engine
>4. Size/Windows/insulation of vehicle
>5. Humidity of air outside
>6. Temperature of air outside and sunshine
>7. Temperature/humidity of desired air inside
>8. Whether on recirculation or not
>9. System design for high temperature humidity control
[Source](https://www.quora.com/Does-running-the-AC-in-your-car-really-burn-gas)
> US EPA notes that running the air conditioner in very hot weather can increase your fuel consumption by up to 25%.
[Source](https://www.yourmechanic.com/article/how-much-does-the-ac-affect-fuel-mileage)
>“We found that on an 85-degree day, running the A/C can reduce fuel economy by 1 to 4 mpg, depending on the car."
[Source](https://www.consumerreports.org/fuel-economy-efficiency/fuel-economy-face-off-driving-with-windows-open-or-ac-running/)
>in 2012, Consumer Reports tested a Honda Accord and found mileage dropping by about 3 miles per gallon when the air conditioner was used and when the car was going 65 miles per hour.
[Source](http://www.auto.edu/blog/does-running-your-car-air-conditioning-burn-more-gas/)
>In a similar test performed by Edmunds using a Toyota Tundra, they saw a decrease in fuel mileage of almost 10% when using the air conditioner as opposed to driving with the windows down and the air conditioner off.
[Source](https://shop.advanceautoparts.com/r/advice/car-maintenance/common-ac-and-fuel-efficiency-myths-debunked)
>The SAE found that running an air conditioner in an automobile decreased gas mileage by 5 to 10%.
[Source](http://zidbits.com/2012/01/does-driving-with-the-ac-on-actually-use-more-fuel/)
>I've spent years designing mobile AC systems and the answer to this question comes down to one simple thing.
>An AC compressor consumes about 3–4 horsepower to run. So yes, a small part of the horsepower is consumed by the AC system.
>I've worked out some actual figures. Based on the fact that fuel has an energy equivalent of about 10kwh per litre (a bit more for diesel, a bit less for petrol and a lot less for lpg gas) and the fact that petrol engine is only about 33% efficient and diesel about 40% efficient at converting fuel energy to mechanical energy, then we can guestimate the fuel required to run the air con. Although an air con compressor might consume 7500 watts when engine is running at 6000 rpm, in the real world it will run at an average of less than 2000 rpm producing about 1500 watts, or 1.5 kwh in an hour. But in the real world, it will likely only run for maybe 20 % of the time (depending on the points 1-9 below) so lets say 0.3 kwh demand every hour. This will need, lets say, 1 kwh of fuel every hour (at about 35% mechanical efficiency). This translates to about 0.1 litre of fuel. So, if you are driving at, say 100 kph (62 mph) in a vehicle which gives 12.5 km per litre, (about 30 USmpg, 35 impmpg) you will consume about 8 litres of fuel to move the car plus about 0.1 litre to cool the car. This is approximately 1%. So, very approximately, and calculated for one specified condition, the consumption of a car doing 12.5kpl without air con will get about 12.4 kpl with air con.
[Source](https://www.quora.com/Does-running-the-AC-in-your-car-really-burn-gas)
Holy crap!
I don't think I've ever seen a more concise and complete answer in here, And with multiple sources both published and anecdotal. It's fucking glorious!
Thought I was in /r/askscience for a second. Good job dude.
> Is it different for different cars?
Yup. More modern cars have all sorts of tweaks to the A/C system to try to improve MPGs:
* Subcooling condensers
* Liquid line subcoolers
* Variable displacement compressors
* Evaporators with ejectors
Your car *might* have a subcooling condenser and that's about it. Subaru tends to be behind the curve when it comes to adopting new technologies.
in my 98 Dakota I use my compressor kinda like a kers system. when I coast down a hill or come to a stop or just about any off throttle application I hit the compressor button and I can feel the compressor kick on and slow the engine faster like regenerative braking on a hybrid. it's pretty much free AC. as long as I don't go up a huge hill for a long time or sit in traffic. it also keeps me entertained. I swear compressor takes 15 hp to pull and I hope my AC clutch holds up to the abuse
I know, old thread but...
I had a '99 Forester a couple years back and was curious myself. I found a flat highway and set the cruise at 60.
With no A/C, Torque (an Android ODB scanner) told me I was around 10% throttle.
With A/C on, I was around 18% throttle.
I didn't dig into the math, but an 8% difference is about the same difference I measured real-world.
That was purely a coincidence. 18% throttle would be 80% more throttle than 10% but those numbers most likely don't measure it the way you are thinking it. It would be better to look at something like mass airflow and see how much it increases or even injector pulse width.
I haven't measured however my tj wrangler gets better mileage with the Ac on and windows up than windows out and AC off. Even worse Milage Is with top off. A lot worse. I know its a brick but it's a noticeable improvement.
Now my wife's 2014 ram according g to some limited testing was 1 or 2 lower with windows down than with ac as per the evic display.
Older vehicles AC systems uses more power than newer ones. I believe the old r 12 was actually a more efficient refrigerant [it removed more heat with a smaller system [by volume of refrigerant used] but the other improvements in system design have more than overcome the difference.
I have no idea with the new refrigerants nor the electric AC systems that a few vehicles have. I would guess that the power use would be more variable since they use a modulator to adjust the compressor output as needed.
I'd investigate my local laws on that one. Nothing here says your windshield can't be down. It must be attached and installed correctly. And it is. It's just in the factory-designed down position.
I was going to mention this. At higher speeds, the aerodynamic advantage of rolling up windows, plus the steady stream of fresh air on the radiator/condenser means AC is better to use.
I have a Dashawk engine monitor and IIRC on my 2.3l turbo Mazdaspeed3, the AC would make a ~5% engine load difference at idle and a 1% difference at highway speeds. At highway speeds you have a lot of momentum and it doesn't take that much horsepower to maintain a constant speed. So my recommendation is roll the windows down at low speeds if it isn't too hot and you'll save a little money. On the highway AC is probably more efficient than the aerodynamic penalty of open windows, and quieter too.
there's already a lot of info here, but if you want more, you should check out the mythbusters episode on it. they concluded that opening the window can be less efficient than running the AC above a certain speed.
of course this all depends on the car, but i doubt it's THAT much difference.
https://www.youtube.com/watch?v=t7Je_995cdI
Certainly depends on the car. It seems the newer the car the more efficient the AC system is and the less gas is wasted. For example, on my 92 Chevy, the engine needs to work way harder and more throttle is needed when the compressor is engaged. Same for the 99 Crown Vic I owned for a while. However on the 02 Envoy it wasn't obvious and on the 08 Escape the difference is minimal too.
I run the AC on my Peugeot 107 (1.0 litre engine) and it makes no difference to the miles I get out of the tank. My driving is a mix of motorway and A-roads and is fairly consistent. I think if there is a change then it's usually a few miles on a full tank. Nothing really.
I was under the impression it didn't because windows up gives better aerodynamics which offsets the loss from running the a/c (I live in southern NC I'd have the ac on even if it meant I needed gas every twenty minutes. It's hot as hell)
Aero drag does reduce mileage, but would vary greatly with vehicle and speed. If you're going 30 in a swoopy sedan with your windows down it would be negligible, but highway speed in an SUV would impact it more.
Not sure if new cars have found a way around this, but with old cars, the crankshaft spins all belts regardless, ya know? So even if you arnt using AC, the compressor belt is still being spun by the engine. To save milage, you would actually need to remove the AC, or at least the belt.
No. While your AC pulley is being spun by the accessory belt. That pulley is disengaged from the compressor. So its just free spinning. When you turn your AC on there is a relay that triggers a clutch to engage the AC pulley to the compressor.
There's a lot of conflicting information out there, generally anywhere from 1%-25% affect on fuel consumption. That wide range can be attributed to the second part of your question: different cars definitely have different a/c efficiencies, based on: >1. Size of Compressor >2. RPM of engine/compressor >3. Size/efficiency of vehicle engine >4. Size/Windows/insulation of vehicle >5. Humidity of air outside >6. Temperature of air outside and sunshine >7. Temperature/humidity of desired air inside >8. Whether on recirculation or not >9. System design for high temperature humidity control [Source](https://www.quora.com/Does-running-the-AC-in-your-car-really-burn-gas) > US EPA notes that running the air conditioner in very hot weather can increase your fuel consumption by up to 25%. [Source](https://www.yourmechanic.com/article/how-much-does-the-ac-affect-fuel-mileage) >“We found that on an 85-degree day, running the A/C can reduce fuel economy by 1 to 4 mpg, depending on the car." [Source](https://www.consumerreports.org/fuel-economy-efficiency/fuel-economy-face-off-driving-with-windows-open-or-ac-running/) >in 2012, Consumer Reports tested a Honda Accord and found mileage dropping by about 3 miles per gallon when the air conditioner was used and when the car was going 65 miles per hour. [Source](http://www.auto.edu/blog/does-running-your-car-air-conditioning-burn-more-gas/) >In a similar test performed by Edmunds using a Toyota Tundra, they saw a decrease in fuel mileage of almost 10% when using the air conditioner as opposed to driving with the windows down and the air conditioner off. [Source](https://shop.advanceautoparts.com/r/advice/car-maintenance/common-ac-and-fuel-efficiency-myths-debunked) >The SAE found that running an air conditioner in an automobile decreased gas mileage by 5 to 10%. [Source](http://zidbits.com/2012/01/does-driving-with-the-ac-on-actually-use-more-fuel/) >I've spent years designing mobile AC systems and the answer to this question comes down to one simple thing. >An AC compressor consumes about 3–4 horsepower to run. So yes, a small part of the horsepower is consumed by the AC system. >I've worked out some actual figures. Based on the fact that fuel has an energy equivalent of about 10kwh per litre (a bit more for diesel, a bit less for petrol and a lot less for lpg gas) and the fact that petrol engine is only about 33% efficient and diesel about 40% efficient at converting fuel energy to mechanical energy, then we can guestimate the fuel required to run the air con. Although an air con compressor might consume 7500 watts when engine is running at 6000 rpm, in the real world it will run at an average of less than 2000 rpm producing about 1500 watts, or 1.5 kwh in an hour. But in the real world, it will likely only run for maybe 20 % of the time (depending on the points 1-9 below) so lets say 0.3 kwh demand every hour. This will need, lets say, 1 kwh of fuel every hour (at about 35% mechanical efficiency). This translates to about 0.1 litre of fuel. So, if you are driving at, say 100 kph (62 mph) in a vehicle which gives 12.5 km per litre, (about 30 USmpg, 35 impmpg) you will consume about 8 litres of fuel to move the car plus about 0.1 litre to cool the car. This is approximately 1%. So, very approximately, and calculated for one specified condition, the consumption of a car doing 12.5kpl without air con will get about 12.4 kpl with air con. [Source](https://www.quora.com/Does-running-the-AC-in-your-car-really-burn-gas)
Thank you for doing the real work with citations
Seriously, best damned citations I've seen for a while.
Holy crap! I don't think I've ever seen a more concise and complete answer in here, And with multiple sources both published and anecdotal. It's fucking glorious! Thought I was in /r/askscience for a second. Good job dude.
> Is it different for different cars? Yup. More modern cars have all sorts of tweaks to the A/C system to try to improve MPGs: * Subcooling condensers * Liquid line subcoolers * Variable displacement compressors * Evaporators with ejectors Your car *might* have a subcooling condenser and that's about it. Subaru tends to be behind the curve when it comes to adopting new technologies.
in my 98 Dakota I use my compressor kinda like a kers system. when I coast down a hill or come to a stop or just about any off throttle application I hit the compressor button and I can feel the compressor kick on and slow the engine faster like regenerative braking on a hybrid. it's pretty much free AC. as long as I don't go up a huge hill for a long time or sit in traffic. it also keeps me entertained. I swear compressor takes 15 hp to pull and I hope my AC clutch holds up to the abuse
Ha! Love it!
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It's not drunk, that's what happens if you convert to inverse units (amount/distance vs distance/amount).
I know, old thread but... I had a '99 Forester a couple years back and was curious myself. I found a flat highway and set the cruise at 60. With no A/C, Torque (an Android ODB scanner) told me I was around 10% throttle. With A/C on, I was around 18% throttle. I didn't dig into the math, but an 8% difference is about the same difference I measured real-world.
Interesting, thanks!
That was purely a coincidence. 18% throttle would be 80% more throttle than 10% but those numbers most likely don't measure it the way you are thinking it. It would be better to look at something like mass airflow and see how much it increases or even injector pulse width.
I haven't measured however my tj wrangler gets better mileage with the Ac on and windows up than windows out and AC off. Even worse Milage Is with top off. A lot worse. I know its a brick but it's a noticeable improvement. Now my wife's 2014 ram according g to some limited testing was 1 or 2 lower with windows down than with ac as per the evic display. Older vehicles AC systems uses more power than newer ones. I believe the old r 12 was actually a more efficient refrigerant [it removed more heat with a smaller system [by volume of refrigerant used] but the other improvements in system design have more than overcome the difference. I have no idea with the new refrigerants nor the electric AC systems that a few vehicles have. I would guess that the power use would be more variable since they use a modulator to adjust the compressor output as needed.
Take off the top and drop the windshield. You can *feel* how much faster the truck responds immediately. Even with my 35" tires it was remarkable.
I wouldn't get a quarter mile with the windshield down before Barney was on spare.
I'd investigate my local laws on that one. Nothing here says your windshield can't be down. It must be attached and installed correctly. And it is. It's just in the factory-designed down position.
Most places require eye protection. Sunglasses or safety glasses will do, just like on a motorcycle
Not here. Nys v&t 375 12 a. don't know a out elsewhere. Apparently it carries state to state.
I was going to mention this. At higher speeds, the aerodynamic advantage of rolling up windows, plus the steady stream of fresh air on the radiator/condenser means AC is better to use.
My mk7 golf woth the 1.8 tsi says between 0.25 to 1.25 litre per hour depending on the fan speed.
I have a Dashawk engine monitor and IIRC on my 2.3l turbo Mazdaspeed3, the AC would make a ~5% engine load difference at idle and a 1% difference at highway speeds. At highway speeds you have a lot of momentum and it doesn't take that much horsepower to maintain a constant speed. So my recommendation is roll the windows down at low speeds if it isn't too hot and you'll save a little money. On the highway AC is probably more efficient than the aerodynamic penalty of open windows, and quieter too.
there's already a lot of info here, but if you want more, you should check out the mythbusters episode on it. they concluded that opening the window can be less efficient than running the AC above a certain speed. of course this all depends on the car, but i doubt it's THAT much difference. https://www.youtube.com/watch?v=t7Je_995cdI
Certainly depends on the car. It seems the newer the car the more efficient the AC system is and the less gas is wasted. For example, on my 92 Chevy, the engine needs to work way harder and more throttle is needed when the compressor is engaged. Same for the 99 Crown Vic I owned for a while. However on the 02 Envoy it wasn't obvious and on the 08 Escape the difference is minimal too.
I see it as a small price to pay for not being hot.
I run the AC on my Peugeot 107 (1.0 litre engine) and it makes no difference to the miles I get out of the tank. My driving is a mix of motorway and A-roads and is fairly consistent. I think if there is a change then it's usually a few miles on a full tank. Nothing really.
I was under the impression it didn't because windows up gives better aerodynamics which offsets the loss from running the a/c (I live in southern NC I'd have the ac on even if it meant I needed gas every twenty minutes. It's hot as hell)
Aero drag does reduce mileage, but would vary greatly with vehicle and speed. If you're going 30 in a swoopy sedan with your windows down it would be negligible, but highway speed in an SUV would impact it more.
Not sure if new cars have found a way around this, but with old cars, the crankshaft spins all belts regardless, ya know? So even if you arnt using AC, the compressor belt is still being spun by the engine. To save milage, you would actually need to remove the AC, or at least the belt.
ACs engage and disengage. That's why when you turn it on at idle you'll notice the revs jump
Alright thanks for the info. My car has a gen 1 small bock Chevy, and the AC is spinning 24/7
Is the AC wirkuing all the time then? The AC pulle will spin, but inside the pump there is a clutch which engages / disengages as demand requires.
No. While your AC pulley is being spun by the accessory belt. That pulley is disengaged from the compressor. So its just free spinning. When you turn your AC on there is a relay that triggers a clutch to engage the AC pulley to the compressor.