# Car airconditioning and fuel economy **URL:** \ **Category:** Factual Questions\ **Created:** [November 9, 2015, 8:11pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132 "2015-11-09T20:11:50Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 12, 2015, 2:38pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/21 "2015-11-12T14:38:46Z")
It’s not been tested because… It’s up to each and every individual to learn how to math it out for themselves. Only YOU can tell us how much YOUR A/C affects YOUR car and YOUR mileage. No matter what test, dyno run, EPA estimate, etc, YOU need to calculate YOUR mileage. Is is not common knowledge how to do this? Fill tank. Drive. Refill tank. See how much fuel you added. Compare to miles driven. Repeat enough times with AC on and AC off to cover a wide array of driving conditions, days, temps, areas, etc and you will start to see a pattern. That’s it. Again… seriously. This is the SDMB. This is GQ. This is doable by everyone with a car and AC. A huge data set awaits us. Please move to the general guessing forum otherwise. . ---
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [November 12, 2015, 3:05pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/22 "2015-11-12T15:05:02Z")
> [@Hail\_Ants](#): > > Moisture is added by the respiration of the occupants in the car. The human body is a humidity ‘engine’, ever time you exhale you’re adding moisture to the car’s cabin air. If there are two or more occupants this is significant. Even when it’s humid & rainy outside the cabin air is still often more easily dehumidified by constantly bringing in that outside air (and dehumidifying _it_) rather than recirculating the same small amount within the cabin. Can vary depending on car interior size, number of passengers etc. but this is why many modern cars automatically disable ‘recirculate’ (and _enable_ the AC) when set to ‘defog’. I don’t know what kind of car you drive but my late model cars all have the ability to recirculate and run the AC compressor at the same time. And it does a fantastic job of drying out the air because, just like the AC in a home, the condenser coils have a drip tube that takes the condensed water vapor out of the car. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [November 12, 2015, 3:21pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/23 "2015-11-12T15:21:20Z")
> [@spifflog](#): > > I know this was done to some degree of accuracy by the Myth busters. > > But, I’m amazed that the fed, some college or university, or some car company hasn’t tested this to a nat’s ass. I understand it would change from car to car, but with all the focus on global warming, fossil fuel usage and the like, why isn’t this available? The most likely reason is that nobody wants to be the one to document (to a nat’s, or gnat’s, ass) how much gas is used for air conditioning. Air conditioning is a nice luxury (considered a necessity in some parts) and people do not like to be made to feel bad about using it. In reality, the difference is probably very small, much less than driving on under-inflated tires or other driving habits. And, as the **Philster** has mentioned, this is easily determined, you just have to keep up it. ---
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [November 12, 2015, 3:26pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/24 "2015-11-12T15:26:16Z")
> [@Philster](#): > > It’s not been tested because… > > It’s up to each and every individual to learn how to math it out for themselves. > > Only YOU can tell us how much YOUR A/C affects YOUR car and YOUR mileage. No matter what test, dyno run, EPA estimate, etc, YOU need to calculate YOUR mileage. > > Is is not common knowledge how to do this? Fill tank. Drive. Refill tank. See how much fuel you added. Compare to miles driven. Repeat enough times with AC on and AC off to cover a wide array of driving conditions, days, temps, areas, etc and you will start to see a pattern. > > That’s it. Again… seriously. > > This is the SDMB. This is GQ. This is doable by everyone with a car and AC. A huge data set awaits us. Please move to the general guessing forum otherwise. > > . I disagree with just about everything you say here. With few exceptions each tank of gas one uses is subject to any multitude of environmental differences and driving patterns. Your suggestion of “repeat enough times” may take so many iterations to gain accuracy as to make the experiment futile. Aren’t you also stating that MPG ratings on cars are useless? It’s a correct assumption to state that individual driving patterns and usage can make a difference in mileage but the idea isn’t the accuracy of the MPG rating but its relation to other vehicles’ MPG ratings as a guideline. So a standardized test has some value. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 12, 2015, 6:14pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/25 "2015-11-12T18:14:47Z")
> [@Philster](#): > > That’s it. Again… seriously. > > This is the SDMB. This is GQ. This is doable by everyone with a car and AC. A huge data set awaits us. Please move to the general guessing forum otherwise. > > . **Moderating** And this is a perfectly reasonable question for GQ. General answers can be given even if they may be different for different individual cars. It’s rather ridiculous to suggest that no answer can be given without doing extensive experiments on your own car. Please refrain from junior moderating. Don’t imply that you speak for the SDMB, or for what is acceptable in GQ. Colibri General Questions Moderator ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 12, 2015, 7:08pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/26 "2015-11-12T19:08:28Z")
Having a sense of humor to make a point is now called Junior Moderating? So, a mod, who should know best, takes it down a notch. Duly noted. My answer is the most valid here, but you frown upon its factual content because of it’s delivery and sarcasm. Good to know. It’s ok to demand more of GQ. I’ve been around long enough to know that. I didn’t just get here. Ciao. (In case no one noticed: I’m not a ‘Mod’ and ‘don’t speak for the SDMD’) ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 12, 2015, 7:25pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/27 "2015-11-12T19:25:37Z")
> [@BubbaDog](#): > > I disagree with just about everything you say here. With few exceptions each tank of gas one uses is subject to any multitude of environmental differences and driving patterns. Your suggestion of “repeat enough times” may take so many iterations to gain accuracy as to make the experiment futile. > \*\*Aren’t you also stating that MPG ratings on cars are useless? \*\* It’s a correct assumption to state that individual driving patterns and usage can make a difference in mileage but the idea isn’t the accuracy of the MPG rating but its relation to other vehicles’ MPG ratings as a guideline. So a standardized test has some value. **No.** Repeat enough times: Every time one fuels up, one gets info. I know my mileage. I know my mileage when the winter/summer blends change at the pumps. I know that A/C has a profound effect on high-revving 4-cylinder engines, but minimal effect on long-geared cars with low-end torque and excellent power-to-weight ratios. For example: Notice this push towards turbos in cars? It’s because the EPA tests don’t engage the turbo, so turbos are a way to get unrealistic EPA estimates for fuel economy. People drive the turbo cars and the EPA estimate is out the window. Add load, like an A/C compressor, and the turbo is really engaged and skewing the mileage now. Most people have easy math 101 access to their actual fuel mileage. The fact that this is considered somewhat blunt or rude (I feel this way from responses) is hard to believe. We’re not counting atoms here, so with enough assorted driving in a summer month with the A/C on vs. assorted driving in a month where the A/C is not on can help one draw some conclusions. I am saying it is much, much harder to make general statements about fuel economy because the profile of cars, engines, turbos, hybrids, transmissions, adaptive computers and such is to the point of mind boggling now. Some cars might lose 2%… some 15%. It’s all over the place. 8-speed adaptive gearboxes connected to engines with cylinder deactivation and multiple driving modes, etc. Tiny 1.6 litre engines vs V8 Hemis. We’re have more variables today than ever before. And variables are what prevent general answers. They just do. I get 31 MPG in summer and 29 in winter over many many samples, because the cold air temp outside means my car is running richer when cold (richer = excess fuel being injected) to get the emissions working and that wastes fuel. I run my A/C in winter and hardly ever in Winter… and my MPG goes… DOWN. A cold engine is inefficient, too, in terms of combustion. BAM: ONE THING, like colder weather, throws the whole guess work into chaos. Except I don’t have chaos… because I check how much fuel I add and how many miles I drive. I can draw some conclusions, but still not nail down the final answer to the MPG hit I take because I still have variables to sort out. Imagine talking at higher levels, and not and individual model that someone actually tracks. . . ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 12, 2015, 7:33pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/28 "2015-11-12T19:33:27Z")
Correction, should read: _I run my A/C in summer and hardly ever in Winter…_ ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [November 12, 2015, 7:42pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/29 "2015-11-12T19:42:34Z")
> [@JerrySTL](#): > > I’ve tested 3 cars (Kia Spectra5, Nissan Altima, and Hyundia Elantra) using an ODBII scanner while on a flat road. On those three vehicles, having any combination of windows open was much more fuel efficient than using AC. > > YMMV. 😃 I’ve had the same result with my VW up to between 77 and 80 miles per hour. After that, the lines on the “graph” intersect and the AC becomes more efficient than the windows. > [@Philster](#): > > Having a sense of humor to make a point is now called Junior Moderating? So, a mod, who should know best, takes it down a notch. You weren’t funny and you didn’t make a useful point. ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [November 12, 2015, 8:41pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/30 "2015-11-12T20:41:28Z")
A past car had an instantaneous gas mileage readout. If I set the cruise control and let the mileage settle, the car would settle right at 30 mpg. Turning on the AC costs .75 mpg. I never tried opening the windows or sunroof so I have no idea what that might have been. Now, my car has a cumulative MPG readout, and that’s worthless for experimenting. ---
**Author:** ![spifflog](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@spifflog](https://boards.straightdope.com/u/spifflog)\ **Post date:** [November 12, 2015, 9:16pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/31 "2015-11-12T21:16:41Z")
> [@Philster](#): > > **No.** > > Repeat enough times: Every time one fuels up, one gets info. I know my mileage. I know my mileage when the winter/summer blends change at the pumps. . . Most people have easy math 101 access to their actual fuel mileage. The fact that this is considered somewhat blunt or rude (I feel this way from responses) is hard to believe. > > We’re not counting atoms here, so with enough assorted driving in a summer month with the A/C on vs. assorted driving in a month where the A/C is not on can help one draw some conclusions. I get it. You are much, much smarter than I am. I should be able to figure it out. It’s just math 101. 1+1=2. > [@Philster](#): > > We’re have more variables today than ever before. And variables are what prevent general answers. They just do. . . Ohh wait. Perhaps I won’t be able to figure it out. All of those variables and stuff. > [@Philster](#): > > I can draw some conclusions, but still not nail down the final answer to the MPG hit I take because I still have variables to sort out. Imagine talking at higher levels, and not and individual model that someone actually tracks. Ohh. Ok. It is unknowable after all. I’m so confused :smack: All kidding aside, I’m just saying is that I’m surprised that there isn’t a published test conducted by the fed, or directed by the fed with guidelines. I understand you think we should all just do the simple math ourselves, but with 250+ million cars in the US alone, you’d think there would be some published guidelines. “We have found that for most SUVs, it’s more economical to roll down windows from 0-30 mpa, from 30-40 it makes little difference and from 40 and above it’s more economical to use the AC.” Would it be perfect? No. But I’m surprised that there aren’t some generally accepted rules with this many cars, this much fuel being used and concern over fossil fuel usage. YMMV of course. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 13, 2015, 12:10am UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/32 "2015-11-13T00:10:29Z")
> [@Really\_Not\_All\_That\_Bright](#): > > I’ve had the same result with my VW up to between 77 and 80 miles per hour. After that, the lines on the “graph” intersect and the AC becomes more efficient than the windows. > > You weren’t funny and you didn’t make a useful point. I feel the same way about you. . ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [November 13, 2015, 12:14am UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/33 "2015-11-13T00:14:55Z")
> [@spifflog](#): > > Ohh. Ok. It is unknowable after all. I’m so confused :smack: > All kidding aside, I’m just saying is that I’m surprised that there isn’t a published test conducted by the fed, or directed by the fed with guidelines. I understand you think we should all just do the simple math ourselves, but with 250+ million cars in the US alone, you’d think there would be some published guidelines. “We have found that for most SUVs, it’s more economical to roll down windows from 0-30 mpa, from 30-40 it makes little difference and from 40 and above it’s more economical to use the AC.” Would it be perfect? No. But I’m surprised that there aren’t some generally accepted rules with this many cars, this much fuel being used and concern over fossil fuel usage. YMMV of course. It becomes less knowable if you don’t account for all the variables… so let’s just pretend that we can just eliminate a few hundred variables and call it a day. BAH! Variables. Who needs 'em! There is this general confusion of, “Why is it so complicated? Why hath not some agency revealed the losses?!” Someone chimes in to just hint at the variables, and the answer is, “Nah, it must be simpler! Some agency must be able to simplify it!” NO. It’s more complex than ever. Want to simplify: Division!!! (e.g, 100 miles / 4 gallons is simple math). I mean, if that example isn’t simple math… . ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 13, 2015, 4:34pm UTC](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132/34 "2015-11-13T16:34:41Z")
> [@Philster](#): > > Having a sense of humor to make a point is now called Junior Moderating? So, a mod, who should know best, takes it down a notch. > > Duly noted. > > My answer is the most valid here, but you frown upon its factual content because of it’s delivery and sarcasm. > > Good to know. > > It’s ok to demand more of GQ. I’ve been around long enough to know that. I didn’t just get here. > > Ciao. > > (In case no one noticed: I’m not a ‘Mod’ and ‘don’t speak for the SDMD’) > [@Philster](#): > > It becomes less knowable if you don’t account for all the variables… so let’s just pretend that we can just eliminate a few hundred variables and call it a day. BAH! Variables. Who needs 'em! > > There is this general confusion of, “Why is it so complicated? Why hath not some agency revealed the losses?!” > > Someone chimes in to just hint at the variables, and the answer is, “Nah, it must be simpler! Some agency must be able to simplify it!” > > NO. It’s more complex than ever. Want to simplify: Division!!! (e.g, 100 miles / 4 gallons is simple math). I mean, if that example isn’t simple math… > . **Moderator Note** Ok, I regard much of this as pointless threadshitting. You indicated in the first post I quoted that your were leaving the thread. I am now instructing you to do so. If you can’t provide constructive input, there’s no reason for you to continue to post in this thread, especially with all the snark. Colibri General Questions Moderator [Previous page](https://boards.straightdope.com/t/car-airconditioning-and-fuel-economy/737132.md?page=1)