Maybe a fun question as we enter summer with higher gas prices... Do
most cars get better fuel efficiency with their windows closed and air
conditioning on (better aero dynamics?) or simply with the windows
down (but worse aero dynamics).  I thought maybe there was an article
out there somewhere that could settle a friendly disagreement between
my husband and I.

Hello happytobeme-ga,m

The answer is ? ?It depends.? In stop-and-go traffic it?s better to
open the windows but at highway speeds turning on the air conditioning
gives better mileage. I?ve collected a variety of articles that
explain the energy use of operating the air conditioner vs. the amount
of drag with open windows. I hope this will settle the ?friendly
disagreement? with your husband.

All the best.

~ czh ~



http://www.mercurynews.com/mld/mercurynews/business/industries/automotive/8721060.htm?1c
May. 21, 2004
A/C vs. open windows on fuel efficiency

Q My friend doesn't run the air conditioning, insisting that the air
conditioning in the car uses more gas. I was told by my dealer that
the new cars use a different system and running the air doesn't affect
the gas mileage. Also, how do you get rid of the stinky smell that
comes from the vents? Please help. I'm about to melt!

A Operating the air conditioning (regardless of a new or older system)
does reduce fuel economy by a small amount, as the engine must rotate
a rather stiff-to-turn refrigerant compressor and generate the
electricity (via the alternator) to operate the blower fan and
compressor clutch. On the other hand, driving a modern vehicle with
one or more windows lowered taxes the engine to a greater degree
because of disrupted aerodynamics. So if the choice is windows down or
A/C, flip that switch on!

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http://www.cbc.ca/consumers/market/files/cars/gasmileage/tips.html
Air Date: Oct. 10, 2000

Using an air conditioner in stop-and-go traffic can increase fuel
consumption by as much as 20 percent. Open windows can be a
fuel-saving alternative to air conditioning at low city speeds.
However, at highway speeds open windows increase drag and heighten
fuel consumption.

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http://autorepair.about.com/cs/generalinfo/a/aa062300a.htm
Gas Saving Tips and Suggestions

Cut down on the use of the air conditioning. A/C takes about 8
horsepower to run and those horses eat gas. Our forefathers crossed a
whole country without A/C, I think we can cross town without it. On a
long trip on the open highway, it is better to use the A/C than to
drive with the windows open. The extra drag of open windows on a
vehicle can eat up more fuel than the A/C will.

--------------------------------------------


http://autos.en.msn.ca/advice/standardart.aspx?contentid=4019743&src=News
Fuel-Saving Tips

When you're in slow city traffic, keep the air conditioner off, if and
when possible. Roll down the windows and open the air vents to keep
you and your riders comfortable. That air conditioner is a burden that
uses fuel, and if you're tooling around town, you can see a "very
slight" improvement in gas mileage by keeping it turned off, a
Mercedes-Benz spokesman said.

In contrast, it is better to use air conditioning if necessary, when
travelling at higher speeds on the highway. Open windows increase
aerodynamic drag and, inevitably, fuel consumption with it. Also,
studies by Volvo and other carmakers have shown that a cooler, drier
and quieter cabin can reduce fatigue on longer trips and help a driver
stay relaxed and alert for longer. No to mention the comfort of
passengers, young and older. These tips should help in using your
vehicle's climate control systems wisely.

--------------------------------------------


http://www.tdc.ca/savinggas.htm
Tips to Save Gas and Improve Mileage 

Use your air conditioning only when necessary. Roll down the windows
or open the air vents to keep your car comfortable on those not-so-hot
days when traveling at city speeds.

Keep windows closed when traveling at highway speeds. Open windows
cause air drag, reducing your mileage by 10%.


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better mileage air conditioning vs open window

Great answer with lots of fun articles to read!

This isn't a direct answer...But then again, i am not a researcher, czh-ga is.

Probably, for best economy, one would turn the car vents all the way
open and to "cool" or "blue" while leaving the AC off and the windows
up.

The air conditioner compressor sucks a lot of energy from engines.
Depending on the car, I am guessing around 15, maybe 20 horses. So
much in fact, that some people turn on the AC when descending a steep
mountain to aid in engine braking. Conversely, they turn off the AC
going uphill. My dad and I both do this in the Nevada desert. This
might not apply to automatics, unless you put them in "2" or "3" going
down hills. Someone correct me if I am wrong. All I drive are manual
tranny cars.

Also, my condolences if you live in California. You are paying
European prices for gas now...