Driving around with the rear windows open? - Fuelly Forums

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Old 06-29-2009, 09:40 PM   #1
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Driving around with the rear windows open?

I remember reading somewhere that if one could, driving with the hatch window open can improve fuel economy because it reduces the amount of negative pressure that is being created behind the car. Well today while I was driving around, it was really hot, between 95F and 105F, in a black car with black interior not using A/C and so I opened the front windows as much as I could during speeds of less than 25mph and kept them closed above that.

Well I thought about the pressure thing with the rear hatch windows and so I opened the rear passenger windows about 1 inch on the driver and passenger side, looked at the computer and noticed the fuel economy improved! I couldn't believe my eyes so I closed the windows again and saw it drop, opened them again and saw it go up again. I tried this several times and while I generally saw a consistent improvement in instantaneous fuel economy, the road elevation tended to vary greatly and so it made it really hard to believe what I was seeing.

So as I was driving, doing my errands, I tried the best I could to find a patches of area where I saw the computer was holding its reading, where the elevation stayed the same and the surface was the consistent as well, then I would try my A-B-A testing with the opening and closing of the rear windows for a long enough duration to get a steady reading from the computer. As for why I didn't open the windows more, it's because when I opened them more than an inch, I saw the fuel economy drop and I figure that's because it would create too much drag since I can't open/remove the back window at all, so the incoming air would just smack against that instead of escaping.

Has anybody verified doing this, I mean just opening the rear windows slightly w/o being able to open/remove the back window (above the trunk) at all?
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Old 06-30-2009, 04:10 AM   #2
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I wouldn't recommend it. I remember reading in the manual of my old Pontiac wagon that driving at highway speeds with the back window open created a suction that could pull carbon monoxide into the vehicle.

-Jay
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Old 06-30-2009, 05:44 AM   #3
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I wouldn't recommend it. I remember reading in the manual of my old Pontiac wagon that driving at highway speeds with the back window open created a suction that could pull carbon monoxide into the vehicle.

-Jay
"old Pontiac wagon"

That's the thing though, this is a 2004 Model Volvo so the risk of getting too much pollution seems to be very unlikely. Since you brought up the idea, maybe I'll put a carbon monoxide meter inside of the car (those wall units) and we'll see if it gets too high if any reading at all!
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Old 06-30-2009, 07:02 AM   #4
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I don't think that a newer vehicle would make a difference. No matter how efficient a gasoline engine is, it will still create carbon monoxide as a byproduct of combustion. I do think that the air deflector on the back of the wagon probably helped, forcing fresh air from above the car down across the window, but dad was always paranoid. He always left one of the side windows cracked for ventilation at all times, and he never drove with the rear window open. I can remember it was like a special treat for us to ride with the rear window down. We only did it when driving the car off-road, and low speeds. Mostly through the 4wd trails on the outer banks of North Carolina. Our neighbors owned a beach house down there and we would often go out with them to the secluded beaches that required 4wd access. The car never got stuck though.

-Jay
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Old 06-30-2009, 08:34 AM   #5
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Yeah usually the tail pipe is past the last opening window of a vehicle but with fresh air coming in from the vents I don't think you have anything to worry about especially since the CAT breaks down CO to CO2 to minimize the CO. You probably breath more from the cars tail pipe in front of you than your own.

Interesting results though because I run with the rear side windows open about an inch in the xB because it provides better air flow from the vents without the fan being on and it is not too noisy. The front windows will rattle and slide back and forth in the tracks if they are opened less than half way - a design flaw - so I either have them half way down or closed.

Now popping the rear hatch on the xB would really be interesting . . . probably suck air in the front windows like crazy.
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Old 06-30-2009, 10:41 AM   #6
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IMHO, The location of the tailpipe has little to do with it... We are talking about the vacuum created behind the vehicle as it travels at highway speed. The exhaust gets sucked into this vacuum, so you are essentially dragging a "cloud" of CO behind you. If you were to open the rear window you are allowing this concentrated cloud direct access to the passenger compartment.
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Old 06-30-2009, 11:32 AM   #7
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I imagine that as long as you have air flowing through from open front windows, it will push back out of the car, emptying any stagnant air, preventing CO from entering, and filling the vacuum (which, along with not pressurizing the cabin as originally asked in the first post of this thread, may help FE).

I would not hesitate to have the rear hatch window open at speed as long as I've got front windows open. Considering the vacuum concern, maybe I'd think twice about opening just the rear hatch window.
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Old 06-30-2009, 11:45 AM   #8
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I remember I once opened the rear window at highway speed. The airflow in the car changed dramatically. Air entered through the rear window, and flowed forward, and out the front windows. I actually had some newspaper in the "way back" and it was blown to the front seat.
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Old 06-30-2009, 11:52 AM   #9
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This was my car, without the woodgrain siding...

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Old 06-30-2009, 01:02 PM   #10
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This was my car, without the woodgrain siding...

That's cool that you could open the rear window, but that thing is DIRRRTY! According to the EPA with their detailed chart, pre 1968 cars were allowed up to 87 grams per mile of CO, 1977 cars were allowed 15 grams per mile, 1984 cars were allowed 3.4 grams per mile, all these measurements at the half useful life of the vehicle which is 50K miles. But for vehicles at the end of their useful life which is 100-120K miles, max CO allowed from 1994-2003 is 4.2 grams per mile, and from there it drops depending on the BIN you choose.
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