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AC vs windows open mileage test

Old 08-15-2005, 09:06 AM
KLCarch's Avatar
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Default AC vs windows open mileage test

Ever since I saw the 'mythbusters' guys do an AC vs windows test I've wanted to do one with my hybrid. They ran 2 expeditions around a track at 45mph: one with all the windows wide open, and one with AC and closed windows. the windows open car got better mileage. But, they weren't travelling at real highway speeds, so I've been wondering how much open windows really hurt.

So this past saturday I had the oportunity to do a highway run where it was fairly flat (unlike where I live). I had to pick my kids up in Corning, NY. The highway there- Rt 17 runs for 50 miles between Binghamton & Elmira pretty flat. there are some slight grade ups & downs, but not much.
I ran 3 rounds of each venting option- driving at mainly 60mph- but varying slightly between 55-64mph with the grade. I figure 3 rounds each way would average out grade & wind variations. roads were dry, temp was 85. rt 17 is a divided 4 lane in this area (2 each way). speed limit was 65; I was passed, but traffic was light, and mostly going the limit, not above. going west, I had a suburban about 10 car lengths in front of me as a windbreak. he was travelling at 60mph.
3 venting options: AC w/ windows closed; no AC- windows closed; no AC diagonal windows- 1 each side of car- open 6"

Driving West - 2 adult passengers:
AC no windows: 56 52.6 56.7 avg: 55.1

no AC/ no windows: (hot!) 64.7 62.8 59.4 avg: 62.3

no AC/ 2 windows venting 65.1 55.6 55.6 avg: 58.7

Driving East - 2 adults, 2 kids:
AC/ no windows: 54.1 54.1 52 avg: 53.4

no AC/ no windows: 63.2 55.6 57.9 avg: 58.9

no AC/ 2 windows venting: 57.1 60.0 58.6 avg: 58.5

overall mpg averages:

AC/ windows closed: 54.25mpg
no AC/ windows closed: 60.6mpg
no AC/ 2 windows venting 6": 58.6mpg

thus confirming the mythbusting theory to me: AC takes alot of energy & is less efficient- at least with these small engined hybrids. I did these test on flat terrain, but I know with my usual driving over hilly roads, the car really works to go up a hill and run AC. I don't use AC much myself anyway- but I'm in the northeast & it doesn't get over 90 much here.
perhaps 40 years ago, when test were first run for fuel efficiency with AC, the bigger powerplants were more efficient with the windows shut and AC, as well as larger window opening volumes and a generally less aerodynamic form overall.

on a side note- this is my first full tank since the recall- I had my oil changed to mobil 1 as well & I got my best tank ever: 55mpg. I love this car- it just keeps getting better & better. I DREAD the return of really cold weather this winter!

Old 08-15-2005, 09:58 AM
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Default Re: AC vs windows open mileage test


thanks for the research. Looks like you covered most of the variables. I have always suspected that driving on the highway with the windows open would be as bad or worse as using the a/c, especially on recirc (vent closed) mode which allows the compressor to work less.

I have also wondered what the effect of drag caused by the rear window is. The old station wagons had rear windows that retracted completely, taking that out of the equation so they probably did a little better. I wish the rear windows of the Escapes could open and eliminate this drag, which at highway speeds has to be huge (driving with the side windows open that is). If you were able to vent at the rear of the car to reduce the pressure inside the car I think the mileage difference would increase with the windows open mileage improving.

I recently removed my roofracks and definitely noticed an increase in my highway mileage. The jury is still out on the city driving - I don't think it would make much difference at lower speed. Although I am looking at my best tank yet on the '06.
Good info. Thanks.
Old 08-15-2005, 10:03 AM
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Default Re: AC vs windows open mileage test

Kate, nice job on the testing....

I had suspected as much, because the real-time MPG meter just goes WAY DOWN when I turn on the A/C, and correspondingly, I have not really EVER noticed it drop significantly when I merely rolled my windows down.

Living in Phoenix, this summer has been WAY HUMID ( for us, anyway ) and the A/C in my car has been on far more than last year, and my lowered MPG is a testament to that, dropping about 3-5 MPG per tank average this summer versus last because or using the A/C more. I have used the windows when I can, but the opps have been fewer this summer......
Old 08-15-2005, 10:07 AM
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Default Re: AC vs windows open mileage test

Hi KLCarch:

___Nice post with excellent detail. Now I know why you are a hypermiler

___The larger the ICE, the less parasitic losses an A/C unit will have on it. The HCH’s smallish ICE takes a pretty big hit with A/C. As long as you know the actual hit, you can decide when A/C is mandatory and or when it is optional. I bet you will be using it more as an option from now on

___Good Luck

___Wayne R. Gerdes
___[email protected]
Old 08-15-2005, 10:07 AM
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Default Re: AC vs windows open mileage test

Mythbusters ----- BUSTED... NOT!

Great project and report Kate, thanks!

If I have time I may try to replicate your test on my 05 Prius, since it is supposed to have a somewhat more efficient compressor and AC system. However, I would guess that no matter what, AC will use a lot of power and thus decrease mileage.

Once again thanks for a great post!
Old 08-15-2005, 01:54 PM
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Default Re: AC vs windows open mileage test

Just as a quick note. I borrowed a HCH from my company motor pool and drove to Southern Pines (near Pinehurst) NC from Chattanooga, TN. I went through Knoxville, Asheville, Winston-Salem on the way. For starters, this is a most Hybrid unfriendly route, next I had my boss and he is used to a Jeep that gets considerably less gas mileage.

On the way over my boss drove 1/3 to 1/2 the way. When he took over the display read 46.1, when we filled up it was around 43. On the way back I drove the entire time and when we filled up the display and calculation was 46.1. I realize this is not good mileage, but considering it was traveling 60-80 (at least on my part) with the air conditioning on the entire time, it was not too bad.

If felt great to get home and jump into a HCH that displayed 55 though .

Old 08-15-2005, 06:03 PM
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Default Re: AC vs windows open mileage test

People got similar results with insights over at insightcentral.net
" no AC/ no windows: (hot!)"
Karen, if you had the same weather we had here (MA), this is dedication above and beyond the call of duty! I wimp out and use AC when the temp get above 80 (mid 70s if its humid)
Thanks for the experiment.
Old 08-15-2005, 06:32 PM
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Default Re: AC vs windows open mileage test

This is great info.

I wonder what the effect is with windows wide open? How much more of a hit that is, than the 6" that Kate tested?

Speaking of windows open -- has anybody noticed how unbearable it is to drive the HCH at highway speeds with just the back windows open (all the way)? It sounds like the car is going to burst. Spooky. The only way to offset it is to open the front windows a bit also.
Old 08-16-2005, 06:49 AM
Join Date: Mar 2005
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Default Re: AC vs windows open mileage test

Hey Kate...

Great post. I live in Syracuse, NY and know that trek well. I used to drive from Long Island to Ithaca, NY and sometimes took Rt 17 between Binghamton and Owego, but have also gone into into the Elmira-Corning area too.

We appreciate your information and post.

Old 08-19-2005, 04:37 PM
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Default Re: AC vs windows open mileage test

Nice testing, thanks.

I usually drive windows up no AC. I hate having the windows down for any reason. My hair blows in my face and you can't hear anything so the middle mileage option is out for me. I'll take window up vent only or windows up AC running every time.

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