[poiney]

When I turn my front defroster on, it always forces the mode for non-recirculated air. Is there a way to keep it on recirculating mode (normally I couldn't care except for allergy season).

[Potty_Pants]

there is an in cabin air filter...

if i was a designer of the car I'd have it active 100% of the time... if that is the case I'd hope the pollen would be collected on it.

now my girfriend is not the brightest... to save a dollar on gas... she'll roll down the windows... and she has major allergies... DUH


gl
ppp

[poiney]

I guess by the lack of responses I can infer there is no solution and it's sneezing vs. fogged up windows.

p.s. if she's trying to do her part to counter global warming than kudos to her!

[jfulcher]

There was a study somewhere that shows that rolling the windows down uses more gas than using the A/C because of the effect the windows down has on the aerodynamics.

[zenmaster]

Originally Posted by jfulcher

There was a study somewhere that shows that rolling the windows down uses more gas than using the A/C because of the effect the windows down has on the aerodynamics.

The study: http://www.sae.org/events/aars/prese.../2004-hill.pdf

Summarizing:
For a sedan, 20% less fuel economy for windows down, starting at 30mph.
5-10% less fuel economy whenever A/C is on.

[zenmaster]

Originally Posted by poiney

When I turn my front defroster on, it always forces the mode for non-recirculated air. Is there a way to keep it on recirculating mode (normally I couldn't care except for allergy season).

Isn't the defroster also a defogger, which would be relatively ineffective if it was using recirculated (interior) air?

[poiney]

Originally Posted by zenmaster

Isn't the defroster also a defogger, which would be relatively ineffective if it was using recirculated (interior) air?

I guess we'll never know! I think that its generally easier to defog by introducing air, but heat on the window will reduce relative humidity and reduce condensation. So I would prefer to have that option.

[poiney]

Originally Posted by zenmaster

The study: http://www.sae.org/events/aars/prese.../2004-hill.pdf

Summarizing:
For a sedan, 20% less fuel economy for windows down, starting at 30mph.
5-10% less fuel economy whenever A/C is on.

Since you quoted the source, maybe you can explain it. At least at ~ 30C, the graphs on pages 6, 7, 14 and 15 all show that AC wastes more gas, which seems more reasonable. On pages 14, 15, the Gal/mi shows a dip, which makes no sense to me, but the overall trend seems correct.

Of course, at colder temperatures, AC will run more efficiently, maybe more so than with the windows down. But then again, this is not so relevant since AC gets more use on a hot day.

In any case, the summary of the presentation, as you mentioned, seems to contract the data.

[zenmaster]

Originally Posted by poiney

On pages 14, 15, the Gal/mi shows a dip, which makes no sense to me, but the overall trend seems correct.

My guess is that the improvement from 50-80 (km/h) is probably due to the way the car was geared relative to the amount of RPMs being generated. For a particular car, each gear has an optimum RPM as far as fuel efficiency. In other words, you can't maintain a car at all speeds (horz axis) and also obtain the car's best fuel economy (vertical axis), because there are only a finite number of gears.

After a certain speed (80km/h), you get the new factor of drag becoming significant. That speed might also be the start of another gear shift.

[poiney]

I agree that gearing seems to be the only obvious factor. Unfortunately, there is no scale on the y-axis and the origin is probably not zero (lame graph), otherwise the effect of gearing is humongous.

BTW, not that Mythbusters is the authority on technical matters like this, but they did an episode and I believe their conclusion was also that for typical scenarios, it was generally more economical to roll down the windows.

[bmw0perat0r]

Originally Posted by jfulcher

There was a study somewhere that shows that rolling the windows down uses more gas than using the A/C because of the effect the windows down has on the aerodynamics.

+2 Head this on Fox News and CNN





I don't really care, I drive so much - wasting gas is a moot point. Whatever amt of fuel you save - I use up.