Mythbusters' mistakes in AC vs windows down episode
Recently, I wrote an article on the debate; do you get better mileage with AC on and windows up or AC off and windows down? A comment in the article said a Mythbusters episode found you could get better mileage with AC off and windows down. The way Mythbusters tackled the myth in that episode was just horrible. The myth was revisited in another episode, which I ordered and will review soon. So, what did they do?
They got an SUV and attached a sensor that estimates mileage by measuring air flow to the engine. They did 15 laps around a track, with a constant speed of 45 MPH, measuring mileage as they went. They did 5 laps with the AC off and windows down. 5 laps with AC off and windows up. 5 laps with the AC on and windows up. No laps for AC on and windows down. Here are the results.
- AC off, windows up = 11.7 MPG
- AC off, windows down = 11.3 MPG
- AC on, windows up = 11.7 MPG
They get two SUVs and emptied the gas tanks. Then filled both tanks with exactly 5 gallons of gas. They both drive around the track, same loads, this time at 55 MPH. One SUV has AC off and windows down, while the other has AC on and windows up. The AC on and windows up SUV stopped first. The AC off and windows down SUV went a 15 percent greater distance.
So, if you trust this test, AC burns 15 percent more gas. Windows down is the way to go. They end by saying they did not really trust the first test because it was just an estimate. The second test was full proof and show the best option for saving fuel and keeping cool was windows down and AC off. Where to start with the mistakes? First, you don't change your testing methods because you don't like the results.
It's really the first rule of the scientific method. You have to attack the method of estimating mileage with air flow. You can't just say, it did not do what I wanted, so it's useless. What's the point of an experiment if you can do that? Didn't they look into if air flow could measure mileage before hand? They should do experiments or research on the how sensitive air flow measurement is in estimating mileage changes.
Changing the speed to 55 MPH in the second test is just inexplicable. They had enough problems going to a different way of measuring mileage and not doing enough experiments. In the middle of all of that, why throw in another variable? They really needed to do a lot more trials and be a lot more careful. I really don't have much hope for their methods in the revisit. These are really basic mistakes.
[Source: Mythbusters]
So, if you trust this test, AC burns 15 percent more gas. Windows down is the way to go. They end by saying they did not really trust the first test because it was just an estimate. The second test was full proof and show the best option for saving fuel and keeping cool was windows down and AC off. Where to start with the mistakes? First, you don't change your testing methods because you don't like the results.
It's really the first rule of the scientific method. You have to attack the method of estimating mileage with air flow. You can't just say, it did not do what I wanted, so it's useless. What's the point of an experiment if you can do that? Didn't they look into if air flow could measure mileage before hand? They should do experiments or research on the how sensitive air flow measurement is in estimating mileage changes.
Changing the speed to 55 MPH in the second test is just inexplicable. They had enough problems going to a different way of measuring mileage and not doing enough experiments. In the middle of all of that, why throw in another variable? They really needed to do a lot more trials and be a lot more careful. I really don't have much hope for their methods in the revisit. These are really basic mistakes.
[Source: Mythbusters]
Reader Comments (Page 1 of 2)
Reynald Gonnin 3:40PM (1/27/2008)
Sorry I did a much better test myself:
I am a technician in the automotive industry, so here we go:
1. Make sure the veh had an recent oil change and that it is in good working condition.
2. Connect a scan tool to the EMC of the veh in question.
3. Look for the power train load result about the engine (within most scan tool capability)
4. Drive at 50 Km, then 80Km and finally up to 100Km per hours, with the A/C on then off and do the same with the windows...
Conclusion: you will find that is more economical with the window down below the 80Km/Hr speed.
On the other hand above 100Km/Hr the A/C on and the window up is better. Simply because the air resistance is higher then the load on the engine that the A/C cause.
The scan tool connected to the ECM is so precise about the load that you can see at least 10% diff before and after an oil change, or when you turn on you head lamp, a 30-40% increase when the A/C or the rear window defroster/heated seats are on,they are some of the worse components to have on, don't forget the heated side mirror!
PS: I use a laptop with a software from the following link: http://www.obdscan.net
Cheers
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MikeB 10:51AM (8/29/2007)
Air resistance rises as the *square* of the velocity. A test at 45mph is essentially meaningless, since at that speed most of your friction is not from air resistance.
On the other hand, if you repeat the test at 70mph, you will see an enormous difference. At that speed, the aerodynamics of the vehicle make a much larger impact on mileage, and having windows open really spoils the overall aerodynamics.
Changing the parameters of the test was a mistake, but a mild one. Doing the test at such a low speed was the real fatal error.
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Bill 11:09AM (8/29/2007)
So at surface road speeds (25-45 mph) the air resistence is a negligible part of friction losses and so the windows should be rolled down instead of using the A/C, but kept rolled up at highway speeds? (55-75 mph)
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Kendall Tawes 11:12AM (8/29/2007)
Frankly the first form a testing was poor science in the first place. The whole thing is a mess but when you consider that air resistance is even greater at 55 and that air conditioning should be more efficient at higher speeds the results for the second test is much more definitive. Sure they are not following the scientific method but they were mistaken in trusting an airflow device like that in the first place. I know in my personal experience with both city and highway driving with AC on and off. Off always gets better fuel economy even with the windows down.
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Lascelles 11:51AM (8/29/2007)
If they drove a third SUV in the second test with AC off, windows up, I think it would have shown almost no difference putting your window down. I hope they did this in the revisit but I doubt it. I don't think they know what they are doing.
I have real doubts about the method of emptying the gas tank too. How much gas is in engine, fuel lines, etc? Considering the lack of respect for variables, I would not be surprised if the methods they are using is just unable to measure the changes in mileage caused by the things they are changing.
I am really upset by this because I am a big fan of citizen scientists testing things but the mistakes are so basic, it would not pass at a high school science fair. I have really lost a lot of respect for show. Maybe the revisit will change that...
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MikeB 12:05PM (8/29/2007)
Bill (comment #2):
Correct. At slower speeds, having the windows down is more efficient. At highway speeds, using the AC is more efficient.
There is a crossover speed somewhere in the middle, where both methods are roughly equal, but that exact speed is going to depend a great deal on the vehicle itself. The Mythbusters seem to have picked a test speed that's close to the crossover value, which is why they didn't see much effect.
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Richard Brown 12:48PM (8/29/2007)
Bill (comment number 5) is exactly right. Vehicle coefficient of drag, CD, will influence the speed at which the efficiency crossover occurs. I have always used 45 to 50 mph as a rule of thumb for the crossover speed in my day to day driving to decide whether to have windows down or AC on. Of course in Dallas sometimes windows down just makes it hotter no matter what the speed.
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grimmex 2:09PM (8/29/2007)
Actually, this was covered in a later episode, when it was re-revisited. I believe it was a representative from Toyota that corrected them on this - that basically the test was done at too low a speed for aerodynamics to play a part at all, and that it's better to roll down the windows up to about 45 MPH, and roll up the windows and turn on the AC above 45 MPH.
Episodes like this one are proof that the Mythbusters methodology is in fact fairly scientific. Their methods are just rigorous enough for most of the myths they test, and results are subject to peer review (namely, viewers, of whom many are scientists).
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4bush 2:32PM (8/29/2007)
SoCool:
Solar A/C Provides the desired effect without any parisidic draw from a convenctional A/C Compressor.
We have created the worlds first solar powered air conditioner system (Helios) that fits into a standard sun roof footprint.
Helios Pays for itself! - Save 5-6 Miles/Gallon by not using the engine driven A/C system. Save the environment by preventing the emissions from that 5-6 Mpg per vehicle becoming pollution.
Helios helps prevent harm to children and pets - Helios will keep the internal heat of an automobile at safe levels for children and pets. So accidental cases where children or pets are left in vehicles need not end in tragedy.
Helios is convenient - Imagine leaving your vehicle in the mall car park for several hours on a hot day, and returning to a cool passenger cabin without needing to switch on maximum AC and swelter until it kicks in.
http://www.travelsocool.com/
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Phil L. 2:44PM (8/29/2007)
4bush -
Solar A/C is interesting - but the web site makes no mention of how the sytem actually works, or any other useful technical info. Is any other info available?
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steven 3:05PM (8/29/2007)
Unfortunately no matter WHO does this test, someone on this blog will say "No, that is not right". I tried this test myself last weekend driving from Annapolis MD to Carlisle PA for the "Pruis at Carlisle Show" (Ha!!! Like that's even gonna show up on their schedule!!!)
I went 150 miles (all interstate) with the cruise at 65 MPH with the AC off and windows down and got 34 MPG. I did the same trip back with the AC on windows up at 65 MPH with the cruise and got 31 MPG. It was on the weekend and there was very little traffic, so I almost NEVER had to get off my cruise. I did not stop to pee or eat. Of course I nearly got blown off the a few times even though I stayed in the right lane except to pass. Sometimes trying to save gas can be harmful to your health!
So if you have the same car I do, you get better HWY mileage with the AC off and windows down. Yes, all your points are valid as to why my results are wrong and you can say I didn't get 34 MPG, but until you're paying for my gas, I'm going to get more MPG with my AC off and my windows down.
On the serious side, keep an eye on your auto-climate control. Most new ones ALWAYS run the A/C, winter, summer, hot cold, mild, what ever and this IS reducing your gas mileage, even when you really may not need your AC.
And what's the deal with this Helios guy? Does he have an RSS-based agent the sends comment SPAM to any blog that uses the words AC and MPG in the same post?!?!?! Actually, I'm glad no one really monitors the posts on this or most of mine would get pulled!
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MikeW 3:27PM (8/29/2007)
The McLaren F1 doesn't let you lower the windows above 130mph. (page 139 Driving Ambition)
Does anyone actually lower the windows all the way on the highway?(yes in the city) I lower the rear window all the way (they only go half way down, tumblehome), and I lower the fronts to match the rear. At that level the vortices shedding off the mirrors don't seem to come into the car as much.
So if the windows all the way down increases the Cd 10%, this configuration might only be 5%.
So in something like an expedition, lower the front windows until the leading edge of the glass reaches the corner, and then lower the rear to the same vertical height.
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steven 3:26PM (8/29/2007)
Phil L. You have to dig, maybe they'd get a few more products sold with a better web page designer... Note to SoCool Time to stop using those free layouts from MySpace! (Do they even have a product that is available for sale? Again the web page is not very clear nor are the guy's "posts".)
Go to the download page and click on the text "SoCool" just to the left of "Power Point Presentation".
It does look promising, not to mention cool, but it also looks like you may lose your sunroof.
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Karkus 4:08PM (8/29/2007)
SoCool...
I'm all in favor of solar air conditioning (and have checked into it myself).
However
1) this post seems a bit too much of an advertisement. Is that really appropriate here?
2) You claim a 5-6 mpg improvement.(Under what conditions, for what kind of a car, etc. etc. etc?????)
Yet your website pdf presentation says "up to 6%", and your calculations assume going from 20.3 to 22.8 mpg. That's only 2.5 mpg. These statements don't match up.
Do you have any real world data? I realize that's hard to obtain, but you should at least try.
For example, take 2 identical cars (with mpg calculation displays) one with, one wihout your system. Park both in sun, then drive for an hour, with both climate control systems set to the identical temp, then compare mpg with/without your system. That would make your claims much more credible. Right now the 5-6 mpg number looks like you just pulled it out of a hat (or excel spread sheet with lots of optimistic assumptions.)
I won't buy anything from companies with such misleading and/or unsupported claims.
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Lascelles 4:15PM (8/29/2007)
I forgot to mention this but there are cars with flow through ventilation. There are a number of inventions that use wind to power AC as well.
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Don 6:20PM (8/29/2007)
Any time you drive 70 mph on the highway with your windows down, you're creating drag. It isn't rocket science.
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Lascelles 10:20PM (8/29/2007)
Don,
"Any time you drive 70 mph on the highway with your windows down, you're creating drag. It isn't rocket science."
Yeah and heavy things fall faster than light things... not :D That's the point I am making. They assumed something and fit the experiment around it. Yes, it creates drag, probably, but how much does it impact mileage? The solution may JUST be turn off or DOWN your AC, it really doesn't matter if you put your windows down. It has almost no impact. Like I said in the other article, opening the window is a hassle. I want proof for THAT alone will have any impact at all. The debate ASSUMES it has impact. Edmunds found it did not and it looked like their test could measure mileage to less than 5 percent.
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Los 2:36AM (8/30/2007)
Guys, it's just a TV show.
1. I don't believe (I'll stand corrected if I'm wrong) they ever claim to have "foolproof" scientific methods.
2. The show is cut for entertainment. I mean how scientific is it if they are constantly cutting away to show Adam put something on his head or do a bad impression of Cecil B. Demille?
3. They still put way more time or resources into this than you ever will. If you, Lascelles, or Autoblog as whole doesn't agree with the experiment. Does the scientific method not dictate that you preform your own experiemt to refute the prior evidence? Whichever results can be duplicated are to be trusted?
Just bitching for the sake of Bitching, and having nothing better to write about on a slow news night.
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Dario 10:12AM (8/30/2007)
The max you're going to get out of a sunroof solar cell is about 300W given solar cell efficiency, and that's in the Sahara, at noon. That's less than 0.5 horsepower and let me tell you my A/C compressor sucks up way more than that from my Elantra - one guy tried a dyno and lost 17hp with the A/C on. So I hope that solar AC's compressor is really really efficient...
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Owain Ozymandias Buck 7:58AM (8/30/2007)
I'm a big fan of quick and dirty--it usually tells you all you need to know before looking at things in depth. However, quick and dirty still has to be sound and repeatable.
Why the heck did they use two different vehicles for this latest one? That's not sound at all. I wonder what the variation in mileage is for new stock vehicles. That might show any difference they detected in two vehicles to be not significant at all.
Still like the show, but they really should bring on board a sharp 6th grade student who has the scientific method fresh in his brain!
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