Is using A/C more fuel efficient than opening a window?

  • Thread starter Thread starter Ivan Seeking
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Discussion Overview

The discussion centers around the fuel efficiency of using air conditioning (A/C) in cars compared to driving with the windows open. Participants explore various claims and personal experiences related to this topic, including conditions under which each option may be more efficient, and the impact on driving dynamics.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants claim that using A/C is more fuel efficient at highway speeds due to turbulence and increased air resistance when windows are open.
  • Others argue that for city driving, it may be more economical to open the windows and turn off the A/C.
  • A participant mentions a personal experience of improved gas mileage when the A/C was not functioning, suggesting a potential relationship between A/C use and fuel efficiency.
  • Another participant references a Mythbusters episode that could not test the claims effectively at speeds over 45 mph, raising questions about the reliability of their conclusions.
  • One participant provides data from their car's data logger, indicating a measurable increase in fuel consumption when the A/C is turned on, although they noted no change with windows open.
  • Another perspective focuses on the comfort and communication aspects of using A/C versus open windows, rather than purely on fuel consumption.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the fuel efficiency of A/C versus open windows, and the discussion remains unresolved with no consensus reached.

Contextual Notes

There are limitations in the discussion, including the dependence on specific driving conditions, vehicle types, and the lack of comprehensive testing at higher speeds. Some claims are based on personal experiences rather than controlled studies.

Ivan Seeking
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MIH was asking some questions about cars, which reminded me of the claim that the air in car tires should be changed periodically...because it gets stale.

Also, I was once told that I should never use fix-a-flat in the car tires or they will explode when serviced. The flat fixer allegedly uses a combustible gas that explodes due to sparks caused by the tire machine used at shops.

Moss only grows on the north side of trees.

If you are being chased by a bear, run downhill. Since their hind legs are longer than the front ones, when bears try to run downhill they tumble.

The toilet was invented by Crapper.
Actually, he was the engineer who designed the London sewer system.

...or maybe not
Thomas Crapper: Myth & Reality
http://www.theplumber.com/crapper.html
 
Physics news on Phys.org
The Rubix cube is solvable.
 
We only use 10% of our brains.
Well, this one is probably true with people who believe it.
 
If you step over a sleeping person, they could dye.
 
Claim: Some people spell dye as die.
Status: True! :biggrin:

Here's a good one. Claim: It is more fuel efficient to drive a car with the windows open rather than running the a/c. The opposite claim is also made. It turns out that [having just seen this discussed with a transportation expert] due to the turbulance and increased air resistance caused by open windows, closed windows with the a/c on is more efficient for driving at highway speeds, and for city driving it is more economical to open the windows and turn the a/c off.
 
My favourite urban myth is the one about the creatures spotted walking around Tokyo.

They made a documentary about it:

 
Last edited by a moderator:
Ivan Seeking said:
Claim: Some people spell dye as die.
Status: True! :biggrin:

Here's a good one. Claim: It is more fuel efficient to drive a car with the windows open rather than running the a/c. The opposite claim is also made. It turns out that [having just seen this discussed with a transportation expert] due to the turbulance and increased air resistance caused by open windows, closed windows with the a/c on is more efficient for driving at highway speeds, and for city driving it is more economical to open the windows and turn the a/c off.

Mythbusters did a episode on this. The fact that they could not test (can't remember why) over 45mph sort of voided their conclusion of no difference.

This last summer the AC in my car went out. I noticed a marked improvement in my gas mileage. I did not expect any change but it improved by about 3mpg. This was with short drives and around town. I did not do much freeway driving. Fortunately I had just replaced my driver side master window switch just a few weeks before the AC went out. Not being able to open windows was a pain with AC, I cannot imagine the misery had the AC went out AND I could not open windows.
 
Sir Joseph William Bazalgette designed the London sewage system.
 
"Moss only grows on the north side of trees."

This may be an urban legend if you take the word "only" too literally, but moss does grow preferentially on the north side of trees, at least in the northern hemisphere. Moss prefers damp, shady areas which are more likely to be found where the sun rarely hits. Of course there is nothing stopping it from growing on the south side of a tree, and the word "only" is too strong.
 
  • #10
How come moss growing is an urban legend, it sounds more like a country side legend.
 
  • #11
Borek said:
How come moss growing is an urban legend, it sounds more like a country side legend.

I don't know about the rest of you, but I was led astray by Lassie, on this claim. NEVER trust a crackpot collie.
 
  • #12
Integral said:
I did not expect any change but it improved by about 3mpg. This was with short drives and around town. I did not do much freeway driving. Fortunately I had just replaced my driver side master window switch just a few weeks before the AC went out. Not being able to open windows was a pain with AC, I cannot imagine the misery had the AC went out AND I could not open windows.

I'm not sure why Mythbusters had such a hard time testing this. It took me about 3 minutes. My track car (also a fun daily driven when called upon) has a piggyback computer for re-tuning and realtime data logging.

At 70mph, my data logger showed an injector pulse width of ~15% at full fuel rail pressure with the A/C off and cruise control on. Just switching on A/C causes an increase to ~16% or 17% to maintain the same speed. That's not a trivial difference. I noticed no change with my windows open, actually.

Here's a myth: driving 55mph saves gas.

I get my best mileage at about 80mph... although I would never know since I've never gone that fast on public roads.
 
  • #13
For me it isn't a question of fuel consumption so far as A/C goes.

It's more about what happens when you turn the A/C on versus what happens when you open a window.

Or, gentle ice cold breeze versus mildly cool gust of wind. Not to mention how difficult communication with passengers becomes with the window open.