Influence of window opening vs. A/C use on fuel economy

It has become popular lately to claim that turning on A/C uses less
fuel than opening windows. On a recent round trip to Kansas I used my
handy Scan Gauge to check milage under different operating conditions.
(What else is there to do driving across Missouri and Kansas?)

I checked the calculated fuel economy over approximate ten mile
segments on each tank of fuel. I used the cruise control and the
calibrated Scan Gauge mph measurements. Usually I could go the whole
ten miles without touching the gas or brake. I tried to be as
consistent as possible.

For open windows, I started out rolling them all the way down but
after a couple segments of that, I decided that no one could stand the
tornado effect for long distances so I tried various partial open
positions which improved ventilation without being punishing.
Generally this was the rears open 4 inches and the fronts either
closed or open 3 inches.

The vehicle, unfortunately, was not typical for most drivers: 1998
Odyssey 4 cylinder with a Thule car-top cargo box. Newer A/C systems
and less drag-challenged vehicles may yield different results, but
here goes...


Test 1 8/31/06 76 mph I35 Southbound Hilly
22.8 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 4 19.83 100
Off Full Open 2 19.45 98
Off Part Open 2 18.90 95
On Closed 4 18.24 92


Test 2 9/4/06 75 mph I35 Northbound Hilly
19.7 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 5 21.62 100
Off Part Open 3 21.27 98
On Closed 1 20.90 97


Test 3 9/5/06 75 mph I35 Northbound Hilly
23.4 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 3 20.43 100
Off Part Open 2 20.65 101
On Closed 3 19.27 94


Test 4 9/5/06 73 mph I55 Northbound Flat
21.6 mpg overall (measured)

A/C Windows # Segments Ave. MPG Relative MPG
Off Closed 6 22.22 100
Off Part Open 5 21.68 98
On Closed 5 20.82 94


Overall, weighted by number of segments relative to windows up, A/C
off, fuel efficiency was:

Windows open 98%
A/C on 94%

In this test, turning on the A/C cost three times as much fuel as
opening the windows. There was no indication that it made any
difference whether the windows were wide open, rears down 4 inches or
front and rear both down 3 - 4 inches, but there was limited testing
of this, and the effect was small in any event.

As noted before the vehicle may not be typical, but these were the
results. One further point of interest; shortly after I started one
segment I came upon a speed reduced work zone. I didn't use the
segment in the above calculations but I noted that average speed was
63 mph and average fuel consumption was 26.4 mpg. It appears that the
difference between going 60 and going 75 was about 4 mpg.

 

The Mythbusters did a controlled experiment, with the result being windows
down = a lot better mileage.
http://cartalk.com/board/showthreaded.php?Cat=0&Number=341160&page=6&vc=1

The link above leads to CarTalk's forums. The post notes that their computer
models said the mileage should be practically the same. The real experiment
showed otherwise.

-- R Flowers

 

The Mythbusters did a controlled experiment, with the result being windows
down = a lot better mileage.
http://cartalk.com/board/showthreaded.php?Cat=0&Number=341160&page=6&vc=1

The link above leads to CarTalk's forums. The post notes that their computer
models said the mileage should be practically the same. The real experiment
showed otherwise.

-- R Flowers

 

Windows down messes up my wife's hair.

A few miles per gallon is cheap for my peace of mind.

Just my .02

Spdloader

 

Windows down messes up my wife's hair.

A few miles per gallon is cheap for my peace of mind.

Just my .02

Spdloader

 

(snip)

Interesting information, but when it's 105 when I leave work, I'll use
my AC and just pay the difference. Better than being covered in sweat
and having all the dirt stuck to me.

Average Hi temp for August in my area=100 degrees. Thanks, but I'll
just keep the windows up!!!

 

(snip)

Interesting information, but when it's 105 when I leave work, I'll use
my AC and just pay the difference. Better than being covered in sweat
and having all the dirt stuck to me.

Average Hi temp for August in my area=100 degrees. Thanks, but I'll
just keep the windows up!!!

 

....

You sir are a gentleman and a scholar.

However, the numbers are small enough, say 2%, that on a $50 tank of
gas we're talking $1.00. And you sure can't hear the radio with the
windows open more than a crack at speed.

I do guess the window numbers would be more significant for an Accord
without the antlers and big slab sides, might get it up to, oh, who
knows, 5%?!? But presumably the A/C numbers would be the same, which
would validate the claim which your experiment did not, but would
still be just about $1.00/tank in the other direction.

OTOH, windows would impose less overhead at slow speeds, say in stop
and go, ... oh, where will it end?!

J.

 

....

You sir are a gentleman and a scholar.

However, the numbers are small enough, say 2%, that on a $50 tank of
gas we're talking $1.00. And you sure can't hear the radio with the
windows open more than a crack at speed.

I do guess the window numbers would be more significant for an Accord
without the antlers and big slab sides, might get it up to, oh, who
knows, 5%?!? But presumably the A/C numbers would be the same, which
would validate the claim which your experiment did not, but would
still be just about $1.00/tank in the other direction.

OTOH, windows would impose less overhead at slow speeds, say in stop
and go, ... oh, where will it end?!

J.

 

Great job. Please test next without the luggage on top and replace 4
cylinder with 6.

 

Great job. Please test next without the luggage on top and replace 4
cylinder with 6.

 

Great Post, it's something I always wanted to do but didn't.
Question, during A/C winows closed, was the Vent on Re-Circulate of
Fresh Air Position
Thanks

 

Great Post, it's something I always wanted to do but didn't.
Question, during A/C winows closed, was the Vent on Re-Circulate of
Fresh Air Position
Thanks

 

A/C is essentially a "minutes per gallon" question while driving represents
more of a "miles per gallon" cost. I think it's certain that at low speeds
the efficiency is better with the windows open and A/C off while at high
speeds the opposite is true. Where that changeover point is undoubtedly
varies widely from model to model, and the "high speed" regime may start
above the speed limit for many cars.

When the Mythbusters ran their test they used SUVs which drank a lot of
gasoline anyway and probably didn't suffer much when the windows were open.
In addition, the speed was so low A/C would be a clear loser.

Mike

 

A/C is essentially a "minutes per gallon" question while driving represents
more of a "miles per gallon" cost. I think it's certain that at low speeds
the efficiency is better with the windows open and A/C off while at high
speeds the opposite is true. Where that changeover point is undoubtedly
varies widely from model to model, and the "high speed" regime may start
above the speed limit for many cars.

When the Mythbusters ran their test they used SUVs which drank a lot of
gasoline anyway and probably didn't suffer much when the windows were open.
In addition, the speed was so low A/C would be a clear loser.

Mike

 

Big haired wife?

 

Big haired wife?

 

Let me guess - you're not married?

-- R Flowers

 

Let me guess - you're not married?

-- R Flowers

 

Nope. Just particular.

Spdloader