Gas mileage hits - windows down vs sunroof vs A/C

acura
rsx

#1

Hi. The guys answered a question a few weeks ago from a lady that drove to Florida. During that call, they mentioned that running the air conditioner actually reduces gas mileage less than running with the windows down. Well, that kind of surprised me, as I had always thought that the windows down (or at least partially down) were more efficient than running the AC. Sort of like running with the tailgate up creates a bubble of air which aerodynamically is nearly the equivalent of having a camper shell or cover on the bed, I thought that having a window down would do the same.



Has anyone ever done any qualitative studies on this? What would be the relative hits on gas mileage to:



* run with the air conditioner on (windows up)

* ride with the window down;

* run with the sunroof open;

* run with the sunroof open and the windows partially down?



Thanks,

--vr




#2

The MythBusters did this one. It was one of their better ones from the data stand point although they did not do a sunroof.

Results, as I recall the AC on windows closed beat the AC off windows open.  

Keep in mind each car is going to have variations in how hard a hit the windows open or the AC on is going to give.  You also would have to add in adjustments for your personal driving style and for the driving conditions (highway/city)  BTW they also did a segment on pickup trucks with tail gate up or down.  Tailgate up won.  

Remember that AC is more efficient in today’s cars than in the days when it first came out.

My recommendation. Drive the way YOU are comfortable, and don’t worry about the mileage difference, as it likely is going to be less than you think.

BTW you should be able to use Google to find when the Mythbuster program may be showing these segments on a station you can access or maybe their web site may have more info.


#3

The crossover point is going to be different with each type of vehicle. A Jeep or a boxy van is such an aerodynamic catastrophe that it probably doesn’t make that much difference whether or not a window is open and there probably is no crossover point.

The more aerodynamically clean a car is, the more an open window ruins its aerodynamics and the lower and more pronounced the crossover point is.


#4

Consumer Reports Magazine claims the only thing that makes a significant difference is whether or not the AC is on.

Windows open or closed, according to their testing of multiple vehicles, does not make a measurable difference in fuel mileage, even at highway speeds.

Hard to believe, I agree, but that’s what they claim.

Personally, the noise factor with open windows is important to me. I like windows closed at highway speeds, and consequently I usually have the AC on in warmer weather. I’ll sacrifice a few MPG to protect my hearing.

What?


#5

I remember the tailgate up/down on Mythbusters, and now that you mention it, I also remember the one with AC on/off…But I don’t remember the results being as conclusive.

I take your point about not worrying about the mileage difference, and to be honest, this is the reason I can’t reliably test the mileage from one to the other. I drive in DC area traffic, so I end up having wildly varying “styles.” One day, I can be home in an hour and change, and the very next day, with the same conditions, it will take me 2 1/2 or 3 hours due to something happening on the road.


#6

Consumer Reports is a good organization but I think their data on this one is flawed. For one thing they don’t usually film their results like Mythbusters so this leaves me skeptical. I also have my own personal observations over the years and windows up/AC on is more economical for my vehicles. This scenario also keeps the wind noise down, no mussed hair, the ability to actually hear the radio and being cool and comfortable. I do admit, however, that windows down while cruising at lower speeds on an early spring day or in the fall can’t be beat.


#7

Highway driving, yes you save gas with windows up and A/C on. City driving at slow speed, you save by having it off, since air reistance is very much smaller.

However, drive the way you want.


#8

“Highway driving, yes you save gas with windows up and A/C on. City driving at slow speed, you save by having it off…”

This may explain why Mythbusters and CR have different results. I have not read the CR report, but I’ll be that they did not drive faster than 55 MPH, and may have done a mix of city/highway driving. The difference can’t be so large that it is worth worring about (to me, anyway).


#9

In its own way, this kind of makes sense. Just as when the Mythbusters set up the water tunnel with the pickup and found that the tailgate up creates a bubble of air circulation which forces oncoming air to pass beyond the tailgate, I can see something similar happening with a window down.


#10

One of the reasons I asked this is that my gas mileage seems to vary fairly wildly, and I’m trying to figure out why – wildly is, say, 8 to 10 mpg. My gas mileage has varied anywhere between 31 and 41 mpg. I keep copious records, recording it each time I have filled up for the full year I have owned it.

I know that part of it is the way traffic flows (or doesn’t flow), but I try to leave early enough not to get stuck in traffic. One to two days a week, I still end up getting stuck on the drive home (rush hour starts at about noon on friday), but for the most part, it is consistent.

I also only use two gas stations (one Exxon, the other a Shell, with a rare stop at Costco if I’m there and running low). I know that the station matters more than the grade.

I’m trying to figure out why I’m getting such a variation.


#11

I’ve seen numerous studies on this subject over the years, as well as studies on pickups with various tailgate/tonneau cover/cap configiurations. Many years ago when cars were much lesss aerodynamic, compressors were much less efficient, and cars only had three gears there was a difference. In modern cars the only consistant result has been that the difference is insignificant and varies “all over the map” depending on a number of different variables.

Back in '89 I did my own pickup truck test. For one year I drove 103 miles each way to my job, 1030 miles per week, using my '89 Toyota truck as a commuter vehicle. That summer I drove for a month (if memory serves) with the tailgate up and a month with the tailgate down, carefully tracking my mileage. There was absolutely no discernable difference.


#12

It is likely 1-2% swing in MPG across the different items.

I will only admit my wife and I never turn off the Automatic Climate control system in our Suabru Legacy and the AC blows cold air while we leave the windows down. Double whammy but never really notice a drop in MPG. Actually warmer temperatures yield a 1-2MPG increase for us.


#13

Actually, on hot days in my current car I’ll sometimes drive with the AC running and the windows AND top open. It keeps my legs and middle cool while the fresh air flows through my gray hair. Sounds odd, but it works.


#14

And exactly how much gas do you save if you don’t actually take the condenser belts off? Without that, you’re still doing 95% of the work of air conditioning your car whether or not you’re running the AC.

Ok, so it’s what consumer reports claims, and like any good scientist, they published in a peer reviewed journal, right? We can see their data? No, it’s hidden. Well, they might be telling the truth. Or they might be making a mistake. How will we ever know?

But look at the physics. The physics of the AC is “pretty much” the same whether or not you run the AC. At highway speeds, you do get drag (and I’ll admit to being surprised when the “jellybean cars” were introduced in the 90s because aerodynamic drag was “significant”) if the windows are open.

On the other hand, AAA puts the cost of operating a car at (oh, I forget, I think) 38 cents a mile. Might be 43. You should look this up. Of this, if you get 20 mpg and pay $2.50/gal, 12.5 cents of this is gas. If you can improve your gas mileage by 10% (not likely) you will save 1.25 cents per mile or $12.50/mo (max, based on 1,000 miles a month). Is your comfort worth $12.50 a month? (max) Recall that as the price of gas goes up, there’s a little more money at risk here.