Economy: A/C on or windows down

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  #1  
Old 06-21-16, 10:05 AM
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Economy: A/C on or windows down

Hello
I worked on cars in the 50's -80's.
I worked on ac compressors and explained to my wife how when you touch the ac button, it engages a clutch to allow the belt to run off the crankshaft thus reducing gas mileage.
She says she heard on TV the drag from the windows being opened is a worse consumer of gas. Is it more efficient to run with the windows up and the ac on or, windows down and ac off? Not comfort just efficiency?
Thank you
Peter
 
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  #2  
Old 06-21-16, 10:27 AM
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My understanding is the increase in drag with the windows open is more of a problem with current vehicles than running the compressor. If you're driving a brick, not so much, but current vehicles rely a lot on having a low drag, aerodynamic shape for their fuel efficiency.

That said, I have no numbers to back this up.
 
  #3  
Old 06-21-16, 10:33 AM
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Google the Mythbusters episode where they examined this "myth". I think they found that with a modern car it saves gas at slow speeds (like 35 mph) to have the windoes down but at faster speeds it's actually more economical to have the windows up and AC on.
 
  #4  
Old 06-21-16, 03:11 PM
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Thank you - my wife wins.
 
  #5  
Old 06-21-16, 06:39 PM
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No, she doe snot.
Spend $40 or so for window visors.
Install.
Crack windows down on passenger side just shy of visor width, so that in case it rains droplets do not fly in.
Drive. You will have very nice circular air flow inside the cabin - at no drag.
Also, it de0humidifies cabin inside very fast during rainy fall and spring days, when you have to run blower to remove mist off windows.
That's another saving.
This is why I seldom ever turn ac on.
 
  #6  
Old 06-21-16, 06:47 PM
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Well that may work where you live, but if you live further down south, you'd be turning on the AC more often that not. Just sayin.
 
  #7  
Old 06-21-16, 07:48 PM
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for Shadeladie. By the time I walk to the garage I'm so hot the A/C feels really good just backing it out.
 
  #8  
Old 06-22-16, 04:05 AM
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mathematically

I was trying to figure out what the Auto Assoc. says or some such body so the answer would be objective and not subjective. Maybe I'll still win.
Regardless of where one lives there has to be a statistical answer.
 
  #9  
Old 06-22-16, 04:19 AM
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I'm with Ray.

Next time our daughter is home, I'll do a little experiment with her car ('09 Corolla). It has a real-time mpg display so would be pretty easy to do a little experiment at say 35 & 70 mph to see how it varies.
 
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Old 06-22-16, 04:23 AM
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That would be great. I look forward to it.
 
  #11  
Old 06-22-16, 04:38 AM
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IMHO its a matter of comfort. Does it really matter if you save a MPG or two one way or the other? If it's hot use the A/C! Point being, why even have the A/C if you don't use it. If you want to save mpg's try hyper-mileing. Slow starts and coast as you approach a traffic light. In most cases running a car at 45 to 55 mph is the most efficient speed. And stay in the right lane as not to piss off other driver who could care less about mileage. Driving is not a competition sport to wage against other drivers. Give yourself an extra 5 to 10 minutes to get to your destination and drive at a more leisurely pace. It's amazing how much better you feel as you drive and when you get to your destination.

In terms of comfort I wish they would bring back manual vents at the floor level that can be closed or open with a lever.
 
  #12  
Old 06-22-16, 05:00 AM
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I've never noticed a difference either way. I've even had the A/C on AND the windows down. And I'll still pull 20 mpg out of my '03 Mustang GT. The only "hit" I see on the vehicle when the A/C is on is that acceleration isn't as responsive...there's a bit of a drag on the engine vs. having the A/C off, when the car will "jump" more when I hit the accelerator.

If I start to notice a slip in fuel economy, the first thing I check is the tire pressures. And pretty much every time MPGs slip, the tire pressures have been low.
 
  #13  
Old 06-22-16, 05:11 AM
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AC windows

Maybe the AAA has a definitive answer under any conditions and regardless of vehicle;
Would the same car get better or worse gas mileage with windows up and ac on or windows down and ac off.
That clutch engaging the crank has to pull more on the engine so more gas has to be consumed. The question is above. Would the same car get better or worse gas mileage with windows up and ac on or windows down and ac off.
 
  #14  
Old 06-22-16, 07:30 AM
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Oh c'mon, boys and girls.
It will hit 100 here in PacNW as much as it will in FL. I had porch thermometer show 127 last summer.
That said, I am driving a hybrid and am hypermiler. I also know that ac comp takes up to 25% of gas consumtion. Mythbusters are not really the absolute scientific final judgement either.
But what i do know is that every business out there, AAA included, is geared towards YOU spending more and buying more. Their "data" are always skewed towards that and this is why I trust real life honest opinions, like in forums.
That said, as I am geared towards gas saving, and even have electric ac compressor, which none of you do unless you drive a hybrid too, I AM absolutely limiting ac comp use. As it does reduce mpg. I can only imagine how more drastic it is on conventional cars and do NOT believe that windows cracked 2 inches down for draft create THAT MUCH drag to overcome 25% of gas consumption. It simply does not make common sense, Mythbusters or not.
That said, if I'm creeping in traffic and pass side windows crack does not supply enough draft, I crack down driver side about an inch.
That said, I have even better heat fighting countermeasure.
My cars are full tint. Full. High heat retention film. Full as in - windshield included. It's a bit of PITA in the dark, but my cars do not heat up as fast as non tinted ones.
 
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Old 06-22-16, 07:46 AM
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Full as in - windshield included. It's a bit of PITA in the dark,
Not to mention illegal in many, if not all, jurisdictions.
 
  #16  
Old 06-22-16, 08:40 AM
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So no conclusion about either alternatives except opinion.
Well thank you for the reply.
 
  #17  
Old 06-22-16, 01:07 PM
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Most cars today will show real-time MPG on their driver info display.

It should be an easy thing to confirm on a highway trip with the cruise control activated.

Run along with the windows open for a while and check your MPG.
Close windows and turn on A/C as you are going along and recheck MPG

Report back with findings
 
  #18  
Old 06-22-16, 02:26 PM
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Makes sense and will do. Thank you
 
  #19  
Old 06-22-16, 02:50 PM
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The consensus is: your wife wins.

SAE study of the question:
http://www.sae.org/events/aars/prese.../2004-hill.pdf

Good examination of various studies:********

Pretty thorough summary :
When to Use Your Air Conditioner - Windows down or A/C on -- which is more fuel-efficient? | HowStuffWorks
 

Last edited by Shadeladie; 06-22-16 at 04:27 PM. Reason: Blogs not allowed. Link removed.
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