# I Gas usage in a car

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1. Mar 12, 2018

### doglover9754

My parents were driving over the weekend with the windows open and I came up with another bunch of questions. With the windows open, would you use more gas than with them closed? I figured that when the windows are open and wind is blowing in, the wind is pushing in the opposite direction that the car is going. So, wouldn’t you have to push more on the gas to keep up your speed? Then again, I don’t drive so I don’t know if what I say is true or not.

Going to the next question, if you use more gas with the windows open, would it be better for the environment with the windows closed and air conditioning on, just as bad, or worse?

I could use this information from the second question for a Girl Scout project in my community someday and I hope I can! As a Girl Scout, I strive to make the world a better place (as stated in the Girl Scout Law) in many ways including beach cleanups, community service projects, etc. I would like to use this information in a community service project or even my Gold Award (the highest award given to a Girl Scout) in the near future so it would be greatly appreciated if anyone could answer my questions. Thanks!

2. Mar 12, 2018

### Staff: Mentor

You're understanding the physics here just fine - if the drag forces from air resistance go up the gas mileage will go down and vice versa. And your intuition that the drag is higher when the windows are open is probably sound - but aerodynamics is tricky and complicated stuff, and it's easy to find counterexamples. (For example - and this is utterly bizarre to me - a 1974 Fiat X1/9 has somewhat better gas mileage with the targa top removed and the cockpit open).

So does running the air conditioner use more gas than driving with the windows open? If you really want to know, you'll have to devise an experiment to find out. (Or you could google to see what other people who have tried this have found out, but where's the fun in that?).

Be aware that the drag forces increase with the speed of the car, while the efficiency of the air conditioner is much less sensitive to that speed. So the answers may be different for highway and city driving.

3. Mar 12, 2018

### doglover9754

Thanks for the help! That made so much more sense to me now. About the testing stuff though, I’ll see what I can try but like I said, I can’t drive T-T. Oh well! Testing is still fun! Also going to the first question’s answer, could the effect be changed depending on what motor or engine is being used? I see where you went with that car example though. When you take off the top, there is very little to no air resistance since all the air does is come back out. Maybe the material of the inside of the car is what makes the air “bounce off” then resulting in better gas mileage.

4. Mar 12, 2018

### doglover9754

I’m surprised you didn’t find this an odd thing to ask from a girl. I usually get that when I ask my friends, family, and teachers about these topics. PF is so nice and not judgmental

5. Mar 13, 2018

### lekh2003

The female popular physics community is surprisingly lacking. If we don't fix that, then who will? Everyone is welcome to ask any question.

6. Mar 13, 2018

### sophiecentaur

PF is more or less gender blind. (And one of the most polite forums you will ever find.)
I notice that your avatar is a bit 'genderist' though - lol. Blokes don't tend to go for cuddly kittens so much. You will notice that my user name is a bit 'feminine' and I do occasionally get 'friendly' replies as a result. But in my case, the choice of name was solely based on the name of my boat. I could have chosen less confusing one, perhaps. I am oldish and male.

7. Mar 13, 2018

### Drakkith

Staff Emeritus
What, is my avatar not cuddly?

8. Mar 13, 2018

### Staff: Mentor

The laws of physics don't care if you are a boy or girl, so why should we or they?
That's disappointing.

9. Mar 13, 2018

### sophiecentaur

Even the dung beetle's mother thinks it beautiful my friend.

10. Mar 13, 2018

### sophiecentaur

Old cultures die hard. Women still do the majority of housework, even those in technical jobs. We all have a way to go.

11. Mar 13, 2018

### Staff: Mentor

I'm energy-conscious and I often leave the windows down for low-speed driving and put them up for highway driving. Since I have no idea where the cutoff is, the decision is at least as much about comfort (including noise).

12. Mar 13, 2018

### jrmichler

There is an entire forum for those interested in this question: www.ecomodder.com/forum. They have a number of threads on exactly this topic.

13. Mar 13, 2018

### Staff: Mentor

Not in my home...

14. Mar 13, 2018

### glappkaeft

One fun that could be had is that you probably would run into articles by or about hypermilers, i.e. people with the hobby of figuring out how to drive as efficiently as possible. The lengths they go to are sometimes extreme.

15. Mar 13, 2018

### Outhouse

Myth busters tested this and found that yes more gas was used and less distance traveled under similar conditions.

https://www.bing.com/videos/search?...31D920CA14E1FE4FA18131D920CA14E1&&FORM=VRDGAR

Yes but minimal amounts as a difference.

This difference depends on the vehicle in question. But in any event, the difference so small on those effected, its really non sequitur in the overall picture of environmental conditions.

16. Mar 13, 2018

### jrmichler

I resemble that remark and we hang out at the site I linked above. There are extreme techniques, but the largest mileage benefit is from smooth driving, looking ahead, and anticipating. My truck is there at: http://ecomodder.com/forum/showthread.php/modding-06-gmc-canyon-17070.html. Last year I drove 10,708.4 miles, and burned 301.87 gallons to get 35.5 MPG in that truck.

Successful hypermilers need a real good understanding of physics, especially managing kinetic energy. The popular misconception of driving like a granny can be counterproductive.

17. Mar 15, 2018

### sophiecentaur

I have tried various economy regimes with varied success but the best one for me has been to use the cruise control (first vehicle I have actually owned with one). That could be counter intuitive by several arguments but what it mainly achieves is stopping 'competitive' driving and encouraging good planning with no unnecessary braking. It is also a technique that gives fewer surprises to other drivers. It gives an mpg (UK gals) improvement of two or three - even in urban driving.
The first lesson is to stop 'enjoying' driving and to treat the car as a way of getting from A to B and to stop looking upon other road users as interlopers and threats to your individual liberty. We all know that an increase in average speed of just one or two mph is very hard won.

18. Mar 15, 2018

### doglover9754

UK gals? The UK has a different measurement for gallons than the US? I never knew that. Huh.

19. Mar 16, 2018

### lekh2003

The US has different units for everything.

20. Mar 16, 2018

### doglover9754

Gosh dang it... why do we have to be so different T-T