# Better fuel efficiency: air conditioning or windows down? **URL:** \ **Category:** Factual Questions\ **Created:** [June 2, 2005, 1:42am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482 "2005-06-02T01:42:27Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [June 2, 2005, 1:42am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/1 "2005-06-02T01:42:27Z")
Actually, not so much a question as something I’ve seen bandied about here a few times. Tonight’s Mythbusters actually loaded Jamie and Adam into identical SUV’s filled with 5 gallons of gas and rolled at 45mph around Altamont. The air-conditioned car went about 30 fewer laps – a drop in fuel efficiency of almost 15%, they said. So, if you were wondering… ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 2, 2005, 3:05am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/2 "2005-06-02T03:05:28Z")
> [@Mathochist](#): > > Actually, not so much a question as something I’ve seen bandied about here a few times. Tonight’s Mythbusters actually loaded Jamie and Adam into identical SUV’s filled with 5 gallons of gas and rolled at 45mph around Altamont. The air-conditioned car went about 30 fewer laps – a drop in fuel efficiency of almost 15%, they said. So, if you were wondering… Damn, Mathochist, we don’t get these till a couple of months after you: you could have used a spoiler warning! 🙂 ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 2, 2005, 3:07am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/3 "2005-06-02T03:07:16Z")
One of my pet peeves is posters who quote the whole OP right after the OP. Damn. Sorry. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [June 2, 2005, 3:15am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/4 "2005-06-02T03:15:19Z")
> [@Princhester](#): > > Damn, Mathochist, we don’t get these till a couple of months after you: you could have used a spoiler warning! 🙂 Oh, I didn’t realize there was a release delay… Maybe a moderator could recode? Honestly, it wasn’t all that exciting. Not much to spoil for that segment. Now the segment about the @#%#ER(\*(&@#%&( [CARRIER LOST] ---
**Author:** ![Meeko](https://avatars.discourse-cdn.com/v4/letter/m/e495f1/32.png) [@Meeko](https://boards.straightdope.com/u/Meeko)\ **Post date:** [June 2, 2005, 3:21am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/5 "2005-06-02T03:21:48Z")
I came here just to cite the Mythbusters EP. Its a repeat. I saw the AC/Windows a while back… maybe even last year… (4th quarter) ---
**Author:** ![aeropl](https://avatars.discourse-cdn.com/v4/letter/a/f05b48/32.png) [@aeropl](https://boards.straightdope.com/u/aeropl)\ **Post date:** [June 2, 2005, 3:29am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/6 "2005-06-02T03:29:57Z")
[What gets the better gas mileage: windows up, AC on, or windows down, AC off?](http://www.straightdope.com/classics/a2_393.html) That article is almost twenty years old, so some things might have changed regarding aerodynamics of cars and the efficiency of auto A/C units. I vote for windows down, but of course it varies with the vehicle and speed travelled. ---
**Author:** ![Reeder](https://avatars.discourse-cdn.com/v4/letter/r/c6cbf5/32.png) [@Reeder](https://boards.straightdope.com/u/Reeder)\ **Post date:** [June 2, 2005, 3:31am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/7 "2005-06-02T03:31:32Z")
It was not a true measure of actual driving anyway. No stop and go…no hills. It means squat. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 2, 2005, 5:16am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/8 "2005-06-02T05:16:11Z")
Reeder constant high speed driving is likely to _maximize_ the contribution of air resistance to inefficiency. Air resistance forms a lesser proportion of overall inefficiency in stop/start driving. Further, engines are generally operating with better efficiency at cruising speeds than in stop/start driving, and so would be more efficiently be able to power the A/C. So if anything, compared to stop/start driving, the Mythbusters’ test was biased against the windows down scenario, and biased towards the A/C scenario. Yet the former was found much more efficient. So it doesn’t mean squat. ---
**Author:** ![sewalk](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@sewalk](https://boards.straightdope.com/u/sewalk)\ **Post date:** [June 2, 2005, 5:16am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/9 "2005-06-02T05:16:57Z")
**Reeder** , Why do you think terrain or traffic would make a difference? The factors in question are the aerodynamic drag of lowered windows and the mechanical load of the A/C compressor. Flat terrain and constant speed make for a simpler test and equating the results to real-world conditions isn’t hard. Having both vehicles stop and go at equal intervals should theoretically further favor the windows-down vehicle due to lower average speed and therefore lower drag. Hills would have a similar but lessened effect. I would like to have seen a comparision at speeds more like those seen on open highways, say 65-70 miles per hour. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 2, 2005, 6:22am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/10 "2005-06-02T06:22:48Z")
Sewalk, while I agree with your overall conclusion, you need to think this: > [@sewalk](#): > > **Reeder** , > Having both vehicles stop and go at equal intervals should theoretically further favor the windows-down vehicle due to lower average speed and therefore lower drag. through a little more. Having both vehicles stop and go at equal intervals should theoretically _minimise the drag advantage_ of the windows-down vehicle due to lower average speed and therefore lower drag and _lower relative importance of drag to overall efficiency_. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 2, 2005, 6:24am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/11 "2005-06-02T06:24:20Z")
Let me try that again. Having both vehicles stop and go at equal intervals should theoretically _minimise the drag **dis** advantage_ of the windows-down vehicle due to lower average speed and therefore lower drag and _lower relative importance of drag to overall efficiency_. [/QUOTE] ---
**Author:** ![kimera](https://avatars.discourse-cdn.com/v4/letter/k/f0a364/32.png) [@kimera](https://boards.straightdope.com/u/kimera)\ **Post date:** [June 2, 2005, 7:10am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/12 "2005-06-02T07:10:33Z")
My special friend who races cars demonstrated for me the lost of power that turning on the air conditioning can cause in a car. It was a drop so noticeable that even I, a person who knows nothing about cars, could tell the difference. When he races he turned on the heater because it helps keep the engine cooler. Of course, his car is built for racing so the average driver might have a different experience. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [June 2, 2005, 8:19am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/13 "2005-06-02T08:19:46Z")
Yep you can demonstrate this for yourself in your own car. Just cruise along with aircon off, then turn it on and you can feel the slight drop in power. It is probably more noticeable in a low powered car. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [June 2, 2005, 8:24am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/14 "2005-06-02T08:24:49Z")
What exactly does “windows down” mean? All four windows rolled down? The two front windows? Just the driver? The driver’s window open half way down? Just a 4" or so slit (which is how I usually drive? Because I’d think each position would affect drag differently. ---
**Author:** ![av8rmike](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@av8rmike](https://boards.straightdope.com/u/av8rmike)\ **Post date:** [June 2, 2005, 12:44pm UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/15 "2005-06-02T12:44:47Z")
> [@Alessan](#): > > What exactly does “windows down” mean? > > All four windows rolled down? The two front windows? Just the driver? The driver’s window open half way down? Just a 4" or so slit (which is how I usually drive? > > Because I’d think each position would affect drag differently. Exactly. The Mythbusters didn’t have a control group: a car with windows up _and_ AC off. Therefore, I’d call their test “inconclusive” at best. Incidentally, we just did this about a month ago: [Driving with the windows down?](http://boards.straightdope.com/sdmb/showthread.php?t=315002) That must be why I was getting the _déja vu_ feeling. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 2, 2005, 1:19pm UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/16 "2005-06-02T13:19:42Z")
Problems with this particular Mythbuster experiment are: [ul] [li]Test was done at a relatively low speed (45 mph) which minimize effects of aerodynamics.[/li][li]Large SUVs were used for the test, which further minimizes the effects of aerodynamics.[/li][li]No attempt was made to make sure the two SUVs were tuned identically. They should have switched cars and done the test again, or run both cars under the same conditions to see how similar their fuel consumption is.[/li][li]They emptied the tanks and put exactly 1 gallon, but it’s hard to tell if both were emptied completely or to the same level.[/li][/ul] Also, they did another experiment by looking at the MPG readout from the vehicle computer, which produced the opposite result. This was from one lap at 55mph, IIRC. I’m inclined to believe that this measurement rather than the more elaborate test. ---
**Author:** ![Tom\_Carroll](https://avatars.discourse-cdn.com/v4/letter/t/3be4f8/32.png) [@Tom\_Carroll](https://boards.straightdope.com/u/Tom_Carroll)\ **Post date:** [June 2, 2005, 5:14pm UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/17 "2005-06-02T17:14:48Z")
> [@scr4](#): > > Problems with this particular Mythbuster experiment are: > > [ul] > [li]Test was done at a relatively low speed (45 mph) which minimize effects of aerodynamics.[/li][li]Large SUVs were used for the test, which further minimizes the effects of aerodynamics.[/li][li]No attempt was made to make sure the two SUVs were tuned identically. They should have switched cars and done the test again, or run both cars under the same conditions to see how similar their fuel consumption is.[/li][li]They emptied the tanks and put exactly 1 gallon, but it’s hard to tell if both were emptied completely or to the same level.[/li][/ul] > > Also, they did another experiment by looking at the MPG readout from the vehicle computer, which produced the opposite result. This was from one lap at 55mph, IIRC. I’m inclined to believe that this measurement rather than the more elaborate test. My hair gets all funky when I drive with the windows down. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 2, 2005, 5:18pm UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/18 "2005-06-02T17:18:24Z")
It also depends on the size of the engine. A large (spark ignition, not diesel) engine running at fairly low power is very inefficient due to pumping losses. A small additional load (AC is around 3-5 HP) may just replace some of the pumping losses and have little effect on fuel economy. Owners of 400+ CID vehicles are aware, for example, that towing a trailer doesn’t hurt thier fuel economy much. (it’s bad no matter what) If, instead, the vehicle is an econobox which is using around 70% of peak horsepower to maintain cruise, then the additional load will push the engine into a less effiecient power range. (due to poor vaporization of gasoline at higher intake manifold pressure) ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [June 3, 2005, 2:36am UTC](https://boards.straightdope.com/t/better-fuel-efficiency-air-conditioning-or-windows-down/306482/19 "2005-06-03T02:36:11Z")
It depends on the engine, really. **Kevbo** ’s mostly making a good point. When I had to drive beater econoboxes, the A/C made a huge, huge difference. Now that I’m blessed enough to drive V8-powered vehicles, there’s not an iota of difference. Well, maybe an iota, but if you take notice of iotas, you’re driving an econobox 4-banger anyway.