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Old 03-08-2008, 05:34 AM   #1 (permalink)
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When does having your windows down start to effect your mileage?

I was curious if anyones ever logged any #'s for windows down and when it really starts to effect FE?

I have vent visors on my truck and when on the freeway I have the cool air vent on (no fan) using the cowls pressure to push air into the cab..

I also have my windows cracked slightly in addition to the vent visors..
Here are the visors..

Not my truck, but same exact design..


I orignally bought them because I like a bit of breeze when driving and I dont like getting wet when raining..

Ya think these are helping things a bit vs not having them with just the window cracked open?

I'd just like to hear some opinions or data..
thanks

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Old 03-08-2008, 09:05 AM   #2 (permalink)
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Hi Chris,

Not too sure if there is any effect with those visors, other than the effect of the visor itself (with the window up). I have on occaision in the summer reduced fuel consumption by tuning the amount of opening of the down wind rear window opening. The amount of opening is only about 1/4 inch or so. Any larger than 1/2 inch makes worse mileage.

This is easy in the Prius as they all have power windows and the fuel economy display. Typically, the difference is about 5 mpg in the Prius when running down the road at 60 mph into a steady 10 to 20 mph off-center head wind. So like from 50 to 55 mpg.

The concept is to inject air into the zone where flow seperation might begin due to the off-center flow.
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Old 03-08-2008, 12:31 PM   #3 (permalink)
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Quote:
Originally Posted by donee View Post
Hi Chris,

Not too sure if there is any effect with those visors, other than the effect of the visor itself (with the window up). I have on occaision in the summer reduced fuel consumption by tuning the amount of opening of the down wind rear window opening. The amount of opening is only about 1/4 inch or so. Any larger than 1/2 inch makes worse mileage.

This is easy in the Prius as they all have power windows and the fuel economy display. Typically, the difference is about 5 mpg in the Prius when running down the road at 60 mph into a steady 10 to 20 mph off-center head wind. So like from 50 to 55 mpg.

The concept is to inject air into the zone where flow seperation might begin due to the off-center flow.
Interesting information!

I usually try to keep my windows up unless it's summertime, in which case no A/C means the windows are way down,
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Old 03-08-2008, 07:42 PM   #4 (permalink)
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Chris D. -

Quote:
Originally Posted by Chris D. View Post
I was curious if anyones ever logged any #'s for windows down and when it really starts to effect FE?

I have vent visors on my truck and when on the freeway I have the cool air vent on (no fan) using the cowls pressure to push air into the cab..

I also have my windows cracked slightly in addition to the vent visors..
Here are the visors..

Not my truck, but same exact design..
...
I orignally bought them because I like a bit of breeze when driving and I dont like getting wet when raining..

Ya think these are helping things a bit vs not having them with just the window cracked open?

I'd just like to hear some opinions or data..
thanks
That's why I got them too. My Saturn was good at getting me wet on rainy days, but I liked the window open so that I could manage my window fogging up. I think they are a good compromise because they allow me to postpone the use of AC as long as possible in the summer. In the Good 'Ol Dayz, I used to drive with the windows down all the way on the freeway, but depending on my speed, I may have them open for that one inch that is covered by the vent visor, on up to 4 inches unless I am below "aerodynamic speeds", aka a traffic jam.

I should do a side window tuft test to see what happens when the window is all the way up. I could do it with and without the vent visors. I would cover the top half with tufts so that I can lower the window half way.

CarloSW2
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Last edited by cfg83; 03-08-2008 at 07:53 PM..
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Old 03-09-2008, 01:20 AM   #5 (permalink)
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Quote:
Originally Posted by cfg83 View Post

I should do a side window tuft test to see what happens when the window is all the way up. I could do it with and without the vent visors. I would cover the top half with tufts so that I can lower the window half way.

CarloSW2
Put tufts on the rear windscreen too... You'll probably be surprised how detrimental even cracking the window open is... At least, cracking the window open even a tiny bit on my MKIV Jetta totally destabilized a bunch of the tufts on my rear windshield... Something I wasn't expecting...
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Old 03-10-2008, 08:26 PM   #6 (permalink)
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This has been a topic of discussion amongst Ford Super Duty owners. The general rule of thumb is to run with windows down up to about 35 MPH. Above that, the aero drag penalty exceeds the A/C load and it is better to run the A/C.

I have WeatherTech visors which are similar to your and they give me a third option in some weather conditions. I can run with the windows down to about a half inch above flush with the bottom of the Weather Techs and get decent cooling without the drag penalty. When the window gets lower than a slight overlap, you can feel the drag increase.
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Old 03-10-2008, 08:50 PM   #7 (permalink)
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windows up/down

Personally,I've never noted a difference.I've kept records for over three-quarters of a million miles on different vehicles,and never saw it on the radar screen.I can state,categorically,that an air conditioning compressor will dig into mileage.Popular wisdom has it, that the loss from running the air conditioner,is compensated for by the loss you'd get from air drag associated with "windows down",so its a wash.Prudence would have it that we take everything on a case-specific basis.It's probably up to the individual,to ascertain whether or not the open glass makes a difference or not.Sorry,I guess thats not much of an answer.Good question though!
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Old 03-10-2008, 11:54 PM   #8 (permalink)
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well, It seems that A/C effects smaller cars more than bigger cars, cause the bigger engine takes the load of the A/C better than the smaller car. (which is I think the reason the smaller cars like the Prius got their FE impacted the most than to the big v8 vehicles with the new EPA ratings.)And it doesn't require me to have the windows open, which for a vehicle with huge windows, and a really bad Drag Coefficient to start with, does less damage than with windows open. For city driving, I crack the windows, for highway, vent on.
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Old 03-11-2008, 01:06 AM   #9 (permalink)
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Quote:
Originally Posted by Big Dave View Post
This has been a topic of discussion amongst Ford Super Duty owners. The general rule of thumb is to run with windows down up to about 35 MPH. Above that, the aero drag penalty exceeds the A/C load and it is better to run the A/C.

I have WeatherTech visors which are similar to your and they give me a third option in some weather conditions. I can run with the windows down to about a half inch above flush with the bottom of the Weather Techs and get decent cooling without the drag penalty. When the window gets lower than a slight overlap, you can feel the drag increase.
mite be a worthy deal to do a yarn string wind test?
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Old 03-11-2008, 01:15 AM   #10 (permalink)
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mite be a worthy deal to do a yarn string wind test?
Yeah, I should do one too. I thought I might get around to this but I sabotaged myself a few days ago when I put Rain-X on the windows. The masking tape wouldn't hold the yarn on. Soooooooooo, I need to dirty-up my windows first, .

CarloSW2

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