AutoblogGreen

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Owain, 1. Actually they do say the second test is "full proof." I should have put that in quotes. 2. Then they should put a disclaimer at the start of the show saying Mythcreators and stop using scientists and near scientific methods. 3. I was thinking about it. I believe the Edmunds test though. I don't think opening the window really matters.

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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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