Windows down or AC?

[ggrimaud]

Several times I have seen statements that driving with the AC on is more fuel-efficient than driving with the windows down. This question has existed for as long as cars have had AC.

Today on the Discovery Chanel the Mythbusters tested it on an SUV. (I think GMC). They drove 15 laps around a track at 55 MPH. Once with the windows up & AC off, once with them up & AC on, and once with the windows down & AC off. The numbers they got were:
[list]

They did a second test with two "identical" SUV's, one with AC on, the other with the windows down. They drove at 45 MPG (for safety reasons because the SUVs couldn't handle the track at 55) until they ran out of gas. In the second test the SUV with AC running ran out of gas long before the one with the windows down.

So the tests conflict. The MPG in the first test was estimated by a computer that measured the air intake. I think it was the same type of test used for EPA MPG estimates. They used the same test with the same equipment, the same driver, on the same vehicle on each run. The second test used two different vehicles with two different drivers at a slower speed and assumed that the gas tanks had exactly the same amount of gas. Also they didn't say that they made sure the tire pressure was the same.

The first test was more scientific IMHO. There was a lot of variables not accounted for in the second test. Also the second test was at 45 MPG. Speed would make a big difference.

So who knows? They said the myth was "busted." Because they were not careful enough in the test they used to "bust the myth", I tend to believe that driving with the AC on is more fuel efficient, provided you are going fast; say 55 MPH.

Mythbusters web site here

[removeum]

8) Had they been driving an 04 or 05 Prius with our electric A/C they would have seen the difference........

Ben

[DaveinOlyWA]

actually i did run limited tests on this very theory and it does appear that the air conditioning will use up about 2 mpg over windows up air off. (the air on tests were done during the hottest period we had all summer with setting at 70º and in daylight driving only. a sidelight to this, i noticed that the headlights seem to affect gas mileage too although not as much)

but the open window thing was much tougher to figure out. there were times when the window open didnt seem to matter. and i found that one window open at speeds up to 40 mph also didnt appear to affect gas mileage much.

i will pursue this a bit more when the weather is better because a lot of the data i have is inconclusive and what data i do have that appears consistant i want to verify again. i had what i thought were real differences but someone pointed out (correctly we determined) that only side by side comparisons on distance, route, and direction would lead to good data.

i found that wind direction especially in areas where a breeze is nearly constant and in same direction. (this explained a 2+ mpg difference in round trip legs)

[Starbug]

Personally, I'd rather have the AC on and windows up, regardless of how it effects MPG. I'm sick of roasting in my no-AC Blazer. Windows down doesn't do squat to cool me once the temps go above 75 degrees. *made SURE to have AC when ordering her Prius* AC is the best thing ever invented for cars.

[DaveinOlyWA]

well Star you are in luck... cant get a Prius without it

[DaveinOlyWA]

i just saw the same Mythbusters show and the window down SUV did go an extra 30 laps (15 miles) on only 5 gallons of gas. that makes an additional 3 miles per gallon which tells me they screwed up somehow.

if the SUV's got 11 something then a 3mpg difference would be over 25%.

something aint right here and im very surprized that they didnt question the results more than they did, although they did express their surprize at the unexpected results.

i will say that the slower you go the less the windows open will affect gas mileage and the Prius will be affected differently to simply because of the way the air conditioning is produced. its thermostatically controlled so temp will make a difference too.

[Batavier]

It would be interesting to see how the Prius would be affected by this. Like Dave said, it would depend on the speed of the car, plus I think the outside temperature, wind speed/direction etc.

From personal experience I can say that on a straight road at around 20 °C and 80 km/h (88 on the speedo) I can see the difference in mileage when I lower all four windows, but can't notice any difference using the aircon (set @ 18 °C). Not totally scientific, I know.

Plus even if using the aircon is worse for my mileage than windows down, I'll prefer to stay 'cool and dry' instead of 'deaf and sweaty'.

[DaveinOlyWA]

so do i ... but we have a good two months in the fall where its too warm to not have air if the windows is up but at the same time, a window open works very well (outside temp at 55-70... sounds cool enough but with a sunny day, it can get very warm inside!!)

the window besides providing more than enough cooling power also gives one more exposure to the fresher than normal air experienced around here. because of inversions and wood smoke etc. the best air here is always fall and spring... spring has a lot of allergy components so fall is best. that is why i like the window open... plus i noticed that my car still has the new car smell to it...another reason why i like the window open

[Wolfman]

What they failed to question was the difference in speed. Wind drag is less of an issue at lower speeds, which is where the variance in their results came from. With the lower speeds, the power that the A/C draws off of the engine became more prevalent. With the higher speeds, the air drag on the open windows offets the power drag from the A/C compressor.

On the highway, I keep my windows up, and let the climate control do it's thing. I still avoid the A/C if the outside temps permit. In town, I open the windows up, provided the temps aren't in the 90's or more.

I wanted to question this test on the chat last night. Unfortunately, I had an unwanted date with the alarm clock, so getting a quick turn in the very loaded chatroom was not going to happen.

[altaskier]

They drove 15 laps around a track at 55 MPH. Once with the windows up & AC off, once with them up & AC on, and once with the windows down & AC off. The numbers they got were:
[list]

They did a second test with two "identical" SUV's, one with AC on, the other with the windows down. They drove at 45 MPG (for safety reasons because the SUVs couldn't handle the track at 55) until they ran out of gas. In the second test the SUV with AC running ran out of gas long before the one with the windows down.

So the tests conflict...
The first test was more scientific IMHO.

I tend to believe the second test more. In the first test, they drove 15 laps at perhaps 2 miles per lap, using about 2.6 gallons of gas. If there's an uncertainty of 0.1 gallon in filling the tank (auto shutoff of pump), that's a 0.1/2.6=4% error in MPG or 0.4 MPG error, so the difference between 11.3 and 11.8 MPG is not very conclusive. In the second test, they probably drained a 25 gallon tank so 0.1 gallons of uncertainty in the fill corresponds to 0.4% error in MPG.