[Superbass]

Which do you think gets better fuel economy, windows up and the air condition on or windows down and the air conditioning off?

Assume windows all the way up or all the way down.

 

[chittychittybangbang]

At what speed?

If you're in stop and go, I would think that open windows would be better. If you're on the highway above a speed where drag from the windows exceeds the extra fuel used by the air conditioning, then in that case, AC would be better.

 

[fstjetta]

I think the mythbusters tested this one. It didn't make a big difference wether the windows were up or the air conditioning was on, but that doesn't make much sense to me. If the windows are up you get additional drag. If the air conditioning is on you get additional fuel burn. The only way I think it would work is if they were driving at about the magic speed where the resistance is equal.

 

[BillK]

I saw that show, I think they were using Ford pickup trucks and there wasn't a big difference. It was a long time ago and I would have to see the episode again.

 

[Blackcat]

There was a difference.

Episode 22:

The fundamental flaw in the MythBusters’ test was that the point where the drag becomes powerful enough to inhibit a car’s performance with windows down was inside their 45 - 55mph margin at 50mph. Going less than 50mph it is more efficient to leave your windows down, but going greater than 50mph it is more efficient to use your A/C.

Tests were performed under varying conditions (55 mph versus 45 mph). Also, the 55 mph test was using a computer to estimate fuel efficiency based on air intake, not actual fuel consumption, and showed A/C was more efficient. The 45 mph test consisted of running the tank until it was empty, and showed open windows were more efficient. This experiment--or one like it--is sometimes cited by the Magliozzi Brothers on Car Talk when presented with this question.

more:

http://www.newscientist.com/backpage.ns?id=mg18925432.200

It will vary from car to car, depending on how aerodynamic the car is and how heavy it is.

Another factor to consider is the difference between the ambient temperature and the temperature selected on the air-conditioning controls. In short, it is not possible to quote one speed that is accurate for all cars.

The results need to be interpreted with caution however, as they were sensitive to a variety of factors including external wind speed and direction and ambient temperature. Unsurprisingly, fuel economy at high speed is much better in more streamlined cars

 

[chittychittybangbang]

That's a good point, if you are driving at 60 mph with a 60 mph tailwind, there is no wind resistance on the car so open windows would be better. A truck or pickup has more wind resistance than a streamlined car. I doubt that ambient temperature and air density has a big effect but I'm sure it has some effect.

 

[Adamgolf]

How would a tailwind of 60 mph make no wind resistance? That just means that the back of the car isn't seeing any difference, the bottom and the front of the car have to be moving at least a little bit of air. A tailwind of 60 mph would just make the air being moved very little. The wheels would be spinning very fast, that also creates some wind resistance.

 

[chittychittybangbang]

Uh...you better put that car on a treadmill In theory, a car moving at 60mph in a straight line with a tailwind of 60 mph would see no signifigant wind resistance except for the part like the wheels, driveshaft, etc. The forces cancel each other out.

Think of the air mass like a block of jello moving at 60mph. If you are moving forward at 60mph and the block of jello is also moving at 60mph, then there is no relative motion except for the wheels and other rotating parts.

Here is another way to think about it - if you open the window on the highway and stick you hand out, you would feel relative wind. If there was a perfect tailwind of 60mph, then your hand would feel no wind. There is also no relative wind acting on the car.

Of course in reality, I think that a perfect tailwind of 60mph would still have a very very small effect, the wind flowing under the car, the wind flowing around the wheels, it would all mess up the air moving around the car and it wouldn't act like a block of jello, it would act like a block of jello with little disturbances all around it. BUT the overall effect would be so small that it would be insignifigant compared to the effect that different car shapes would have, etc.

 

[Therion]

I don't get it, you are contradicting yourself. how would theory be different from reality? if the air mass is moving like "jello" at the exact same speed and is cancelling forward wind speed out, the air should be stable all around the car, except ofcourse the wheels.

 

[McJamma]

2nd, if the tailwind was moving at exactly 60 and you are moving forward at 60, there should be no wind restance.

 

[chittychittybangbang]

Let me clarify....there is almost no wind resistance in reality, but the act of the car bouncing up and down on the suspension, the wheels and driveshaft moving, the exhaust and intake air moving about the car, etc. This is so small as to have effectively no effect so I guess you could say that there is no effect but I didn't want a stickler to say that there was...either that or I am stupid and this has no difference between a stopped car and a moving car vs a car with a tailwind acting on it, but my feeling is that it does. In other words, the wind resistance at 60mph with a 60mph is almost very tiny as to have no effect on mileage but the air around the car is not 100% still.

 

[Therion]

So in other words, there is no relative wind?

 

[chittychittybangbang]

I think it's a matter of semantics, there is no "relative wind" in the sense that there is no signifigant airflow but I think that there is some air movement around the car.