Gas Usage in Cars: Effects on Environment

[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!

 

[Nugatory]

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.

 

[doglover9754]

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.

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.

 

[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

 

[lekh2003]

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

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

 

[sophiecentaur]

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

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.

 

[Drakkith]

Blokes don't tend to go for cuddly kittens so much.

What, is my avatar not cuddly?

 

[russ_watters]

I’m surprised you didn’t find this an odd thing to ask from a girl.

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

I usually get that when I ask my friends, family, and teachers about these topics.

That's disappointing.

 

[sophiecentaur]

What, is my avatar not cuddly?

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

 

[sophiecentaur]

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

That's disappointing.

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

 

[russ_watters]

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.

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).

 

[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.

 

[berkeman]

Women still do the majority of housework

Not in my home...

 

[glappkaeft]

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?).

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.

 

[Outhouse]

With the windows open, would you use more gas than with them closed?

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

So, wouldn’t you have to push more on the gas to keep up your speed?

Yes but minimal amounts as a difference.

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?

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.

 

[jrmichler]

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.

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.

 

[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.

 

[doglover9754]

It gives an mpg (UK gals) improvement of two or three - even in urban driving.

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

 

[lekh2003]

The UK has a different measurement for gallons than the US?

The US has different units for everything.

 

[doglover9754]

The US has different units for everything.

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

 

[Stavros Kiri]

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

Because everyone is ...

 

[Stavros Kiri]

Everyone is welcome to ask any question.

Except perhaps "Gee! Is my theory correct?" ...

 

[lekh2003]

Except perhaps "Gee! Is my theory correct?" ...

We can all relate .

 

[sophiecentaur]

The US has different units for everything.

Based on the desire to make people think they can drink more and that their fuel costs even less than it really does.

 

[Tom.G]

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?

If your car is a newer one with a fancy computer, there may be a function to display MPG in near real time. If so, a trip on a deserted road with cruise control turned on could be good conditions to try the 'open windows' versus 'air conditioner' experiment. Try it traveling both directions to average out wind and elevation changes.

And be sure to report the results!

Cheers
Tom

 

[sophiecentaur]

If your car is a newer one with a fancy computer, there may be a function to display MPG in near real time.

I read a comment on PF (a long while ago and I can't remember the context) that the mpg display may not be as reliable as you might think. It is not (apparently) arrived at by measuring the actual flow rate of fuel but by measurement of the engine parameters.
In the 70's there was a craze for mounting fuel flow measuring units in cars and a take-off at the speedo input and that could give reliable mpg results. But it would involve a bolt - on which few people would bother with. (Drilling holes in the dash board is not a popular hobby these days. )

 

[Nugatory]

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

History is why they started different. Tradeoffs between the cost of switching (retooling an entire economy, from the hand tools in mechanics' garages to lathes and milling machines with six-digit price tags, to factory assembly lines with nine-digit price tags) and the cost of being incompatible with your trading partners is what keeps them different. It's not an accident that America is only major economy that hasn't metrified.

And take a look at the first paragraph of https://www.physicsforums.com/threads/gravitational-constant-in-english-units.737311/#post-4655335 :)

 

[sophiecentaur]

It's not an accident that America is only major economy that hasn't metrified.

It could be a consequence of US isolationism at a time, way back when there was a lot of industrial expansion. All those new factories, employing so many workers would not make a sudden change very attractive. It must be a real pain for manufacturers who live on the the Imperial / SI interface - twice as many drawers of nuts and bolts.

 

[russ_watters]

I read a comment on PF (a long while ago and I can't remember the context) that the mpg display may not be as reliable as you might think. It is not (apparently) arrived at by measuring the actual flow rate of fuel but by measurement of the engine parameters.
In the 70's there was a craze for mounting fuel flow measuring units in cars and a take-off at the speedo input and that could give reliable mpg results. But it would involve a bolt - on which few people would bother with. (Drilling holes in the dash board is not a popular hobby these days. )

It's a good chance I was involved in that discussion, but I don't have a lot of sympathy for the manufacturers, if that's their excuse. There are lots of things you can measure that make good proxies for fuel flow and cars are so exquisitely well engineered/sensored/controlled that they know with a very high degree of accuracy what the fuel flow is. That makes it more unforgivable that car trip computers are inaccurate and more likely that it is done on purpose (anyone ever see one with an error low?). To put a finer point on it, you can buy a 3rd party device that reads your computer's telemetry and (among other things) functions as a very good trip computer:

I still use this because it is more accurate and comes with a calibration function (and I'm nerdy like that), whereby it guesses how much fuel you just put in the car and then you correct it. The gallons and correction factor (in %) are then shown. This device tends to run 1-3% high (varies based on weather and driving conditions/style), vs the car's trip computer which runs 10-15% high. Given that they have access to the same information, there is no excuse for that extra 10% or so error.

Note: this is a Kia Optima, a company (not model) that had to mail customers gas cards for false advertising of fuel economy. My model was not included, but perhaps not coincidentally the current advertised fuel economy is about 10% below what it was for the model year I bought (31 vs 34 mpg highway).

 

[sophiecentaur]

@russ_wattersers Time was, in UK when you could just read how many gallons of fuel you bought by reading the meter on the pump. As you drove away, you could do a calculation in your head. For years, now, we (EU) have been using litres for our liquid measure and there is not an approved unit (for comparison) involving litres and miles. So we stick with mpg (instead of the l/100km in Europe) and I have to do the conversion on paper every time on my ancient Hilux truck. With the Brexit situation, I can't imagine UK going over to km or driving on the right for many years so things are not going to get any easier for us tightwads.
Just open your wallet (=billfold?) and say "help yourself".