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Hi, I tried to search the forums for an answer, but couldn't find one so I would like to know at what speed the FE will decrease w/ windows down vs. closed, with respect to vehicle speed (I don't have a Scan Gauge yet). BTW I drive a Focus hatch if that matters w/ aerodynamics. Thanks

I'm no expert, but I'm pretty sure that it hurts your aerodynamics/fuel efficiency at all speeds. I guess that windows open don't hurt much at low speeds. My rule of thumb is that at about 30 MPH I roll them up and use the fan instead. If the dog is with me, they stay open to about 40.:)

If you have a rear sliding window - you might be able to increase air circulation w/ windows rolled up by a factor of 2x-3x. try this:

set air to circulate only at floor - open rear slider

air should blow from floor and get sucked out rear slider at a fairly high rate .

There was an episode of Myth busters where they mentioned that at around 80 Km/H (50 MPH) it is more fuel efficient to close your windows and use the A/C. I keep windows open until about 25 MPH, depending on the driving itself, and I almost never use the A/C (I never use it if I'm alone).

There my heroes (See I am a nerd :-)). What they determine-- busted, plausible, or confirmed??

Though I tend to do about that. At highway speeds (over 40) close windows, makes too much noise anyway.

--des

There was an episode of Myth busters where they mentioned that at around 80 Km/H (50 MPH) it is more fuel efficient to close your windows and use the A/C. I keep windows open until about 25 MPH, depending on the driving itself, and I almost never use the A/C (I never use it if I'm alone).

The only problem with all the comparisons I have ever seen is that they were between A/C and windows down. As a rule of thumb, the vent will be the most efficent at all speeds. Windows-down is 2nd & a/c 3rd at speeds below 30-50mph depending on car. At speeds about 30-50mpg, a/c is 2nd & windows-down is 3rd.

If I'm averaging over about 15/20mph, and the outside air is cool enough, I use the vent fan. In my truck, I used that exclusively unless it just couldn't keep up with the heat since the ICE always generates power so there is an excess, where in the hybrid I want to conserve all electrics that I can. However it may not be necessary...

Anyone know which power source (12v or hybrid battery) the actual vent fan uses on various models of hybrids? My understanding is that on the HyHi, the compressor is run by the hybrid batter, but I never thought about what runs the fans.

I would be skeptical that it would make much of a difference at all at 30-40 mph speeds. There is 1/8th of the aero drag at 30mph that there is at 60mph. Opening one window at 30 simply couldn't make a very big difference when there is so little wind resistance anyway at that speed. This is all seat-of-the-pants, but I roll mine up between 40 and 50 mph, coincidentally about when it starts to get too noisy and breezy.

What leaves me unimpressed by the analyses window down vs. a/c is the assumption that the windows are all the way down, with max drag.

I get cooling by just cracking the passenger window 1/2 inch and opening the drivers by 2-4 inches while driving 30-55. Below 30-25 I may open them all the way. Above 55 I may turn the vent on with fan to 1, but if I simply crack either window a 1/2 inched I can feel the vent flow increase by much more than double.

Only time a/c runs is if the wife is in the truck.:Banane59:

I fortunately work where I'm in and out so I dress warm in the winter and shorts and T in summer so it doesn't matter if I sweat a little.

Oh and lets not forget wind speed...if you are heading into a 40/50+ mph wind, your going to want your windows up even when your crawling. But as a tail wind, you should be able to have your windows down at 60+ without noticable air drag.

TMV - Too Many Variables

I always have a window cracked or open. I never run buttoned up. The vent open and the vent motor running isnt enough for me. I need air flow,,, and lots of it!

worthywads, thanks for the tip, I'll have to try that, I usually have them open all the way or closed shut. I didn't think it mattered much whether it was opened a little or all the way. Did anyone see what diff's there are w/ the scan gauge, I'm curios of how much it is (window cracked~ 1-2in vs open fully)? Thanks

I think not only does opening your windows and drag relate to the speed at which air is flowing by the vehicle, but also how "opened" the windows are. At speeds up to 30 MPH, I'll usually have the windows about 1/2 way opened. Over that, maybe 1/4-1/3. Above 50 MPH, I'll just have them opened a crack (an inch).

Sometimes the vent will suffice, if the outside air is cool/dry enough.

If I roll down just my rear windows on the HyHi or just the top on the SportTrack, I get severe buffetting at certain speeds where the air pressure rapidly changes rapidly between positive & negative. Does that suggest poor aerodynamics at that speed, or would that be vortexes just along the edge of the window & cause minimal additional drag?

My experience (subjective) is that drag starts becoming noticeable somewhere in the 36-38mph range (with one or more windows lowered all the way). I'm stubborn so I tend to run with all windows up with just the fan on speed setting 2 (of 4). During winter anyone would do this. During summer however, most people would rather roll down the window when at slow speeds. Since I refuse to do that at moderate to high speeds and I WILL cook because of this, I prefer to just keep them up all the time so at least I can become acclimated to the temperature.

Obviously, this isn't for everyone but I've had good success with it.

I roll the windows up just before I shift into 5th gear.

I usually put the driver down ~2 inches and the REAR passenger down ~1 inch. That seems to smooth out the flow, so it doesn't feel like someone's trying to wrestle my head on the highway.

One thing everyone should do, even in the winter, and ESPECIALLY in VWs - make sure you roll down ALL electric windows at least monthly. Too often the wires get brittle, cracked, and fail due to lack of use.