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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.
Reply to
Gordon McGrew
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The Mythbusters did a controlled experiment, with the result being windows down = a lot better mileage.

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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

Reply to
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

Reply to
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!!!

Reply to
Bob

...

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.

Reply to
JXStern

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

Reply to
Art

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

Reply to
ottguit

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

Reply to
Michael Pardee

Big haired wife?

Reply to
Elmo P. Shagnasty

Let me guess - you're not married?

-- R Flowers

Reply to
R Flowers

Nope. Just particular.

Spdloader

Reply to
Spdloader

Fresh Air.

Reply to
Gordon McGrew

I am. I am fortunate to be married to a woman who doesn't worry about her hair.

Reply to
Elmo P. Shagnasty

Yeah, you have to factor in the cost of a trip to the hairdresser too!

Reply to
Matt Ion

I'm with you on this one - yeah, there's a MEASURABLE difference, but is it really that NOTICEABLE to the average driver? And is the difference worth the comfort?

It's kinda like the amusement I get from seeing people lining up or going out of their way to go to a gas station that has prices a few cents less per liter - with my 50 liter tank, a two-cent difference means a savings of a whole $1.00, IF I have to fill up from empty. It's just not worth $1.00 of my time to sit and wait in line or to go out of my way.

Reply to
Matt Ion

I thought the compressor worked less when the Vent is not always bringing in Warm air from outside, I wonder what difference that would have made. Personnally though, I worry about having it in Re-Circulate mode on a long trip and not getting enought Oxygen and staying alert. Bg

Reply to
ottguit

I think it brings in some fresh air even in recirc mode.

Regarding your point on the compressor; This car does not have a thermostat but I do sometimes sense that the compressor is shutting off sometimes. Not so much that I can say for sure. Does anyone know if this car cycles the compressor?

Reply to
Gordon McGrew

The difference would be about $2 per tank for this vehicle or about fifty cents an hour. Well worth it when needed - which it really wasn't on this trip.

If this test were done in an Accord or Civic or even the Ody without the roof box, the cost per hour might be similar but larger as a percentage of total fuel cost.

Reply to
Gordon McGrew

Fair'nuff... my Accord gives me usually 500-550km on a tank (80% city driving), which works out to around 28mpg (alright for an '87 with almost 420,000km), so it really isn't worth the difference for me.

Reply to
Matt Ion

System pressure cycles the compressor, by nature of its design.

Spdloader

Reply to
Spdloader

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