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Fuel efficiency of windows rolled down vs. AC on (trip data)

B

b0mbrman

Lifer
Just finished 24 hours of driving early this morning and have the AC v Gas data.

Notes
The thermometer was placed on the passenger seat around where their butt would be
I also kept track of the average outside temperature, cruising inside temperature, as well as locations and time and other garbage.
Avg fuel efficiency with windows down was 29.58 mpg
Avg fuel efficiency with AC on was 27.75 mpg

Anyone not surprised?

Link to all of the data

More garbage
Additionally, I was curious to see how much cooling could actually be done using the windows so during the third leg of the trip when it was approximately 100° outside, I rolled up the windows until it was over 100° inside (took about half an hour). It turns out open windows will eventually lower the temperature inside a car about 9°. Opening the sunroof is good for another half a degree...

Link to that data

Your thoughts?
 
From a quick glance at your data, it appears that the difference in fuel efficiency is not significant. While the averages are different, your range of values overlap a lot. It might be possible to bootstrap the data to see if it's significant (maybe try a 1- or 2-way ANOVA).
 
how high was your ac cranked up? did you have the AC set to keep ~ the same temp as you had with the windows down? otherwise this data is inconclusive.
 
Originally posted by: mordantmonkey
how high was you ac cranked up? did you have the AC set to keep ~ the same temp as you had with the windows down?
Click to expand...

Yup. AC was adjust to maintain an inside temperature of (Outside - 9°)
 
Your AC cruise speed is higher, and sounds like a more on/off throttle route.
The actual difference in mileage probably isn't even that high as you've calculated, I would imagine.

So, after all this, what's better then did you find? Driving in 100 degree heat and getting better mileage, or driving in a nice cold car and getting worse mileage? 🙂
 
Originally posted by: RedRooster
So, after all this, what's better then did you find? Driving in 100 degree heat and getting better mileage, or driving in a nice cold car and getting worse mileage? 🙂
Click to expand...
true true :laugh:
 
I thought during city driving, windows down is more efficient

but on the highway, AC & windows down are basically the same since windows down creates drag
 
I'm not surprised, I've been saying this for a long time. Modern cars are very aerodynamically efficient even with the windows down. They're designed that way in order to reduce wind buffetting noise with the windows up.
 
Originally posted by: DingDingDao
From a quick glance at your data, it appears that the difference in fuel efficiency is not significant. While the averages are different, your range of values overlap a lot. It might be possible to bootstrap the data to see if it's significant (maybe try a 1- or 2-way ANOVA).
Click to expand...

Yeah, I'm not happy with how little isolation there was but there were too many variables I couldn't control. Of note are the reasonably strong correlations between temperature and efficiency as well as miles traveled and efficiency.
 
Originally posted by: RedRooster
Your AC cruise speed is higher, and sounds like a more on/off throttle route.
The actual difference in mileage probably isn't even that high as you've calculated, I would imagine.
Click to expand...

Nope. Just got to a comfortable speed, put in the cruise control, turned on some music, and watched the temperature (and the road from time to time 🙂)

The average speed reflects speed limits: 65 in Texas at night, 70 in Texas during the day, 75 in New Mexico and Arizona, and 70 in California (but police care less)

So, after all this, what's better then did you find? Driving in 100 degree heat and getting better mileage, or driving in a nice cold car and getting worse mileage? 🙂
Click to expand...

I never did this to save money. I just needed something to keep the old brain occupied.
 
Originally posted by: StevenYoo
Mythbusters did a thing on this too.

same results: AC/windows down was less efficient
Click to expand...

That's what immediately popped into my mind too when I saw this thread title. Love that show.

 
Originally posted by: b0mbrman
Originally posted by: DingDingDao
From a quick glance at your data, it appears that the difference in fuel efficiency is not significant. While the averages are different, your range of values overlap a lot. It might be possible to bootstrap the data to see if it's significant (maybe try a 1- or 2-way ANOVA).
Click to expand...

Yeah, I'm not happy with how little isolation there was but there were too many variables I couldn't control. Of note are the reasonably strong correlations between temperature and efficiency as well as miles traveled and efficiency.
Click to expand...

Yeah, I understand. It's impossible to control so many variables, but your analysis looks nice :thumbsup:

Even if the correlation holds following some other non-parametric analysis, it becomes a quality of life issue: I'd give up 2 mpg for some A/C 😛
 
Originally posted by: StevenYoo
Mythbusters did a thing on this too.

same results: AC/windows down was less efficient
Click to expand...

That was the first test based on the cars computer. In the track test the window down was way more efficient. It has something to do with how the computer determines fuel economy something with airfolw across the engine.

Window down is more efficient any way you look at it. Makes sense.
 
That'd depend alot on the car. Larger, torquier engines will have less of an effect from A/C. I don't think a diesel pickup even notices when the A/C is on or not. A smaller car like a Civic or a Metro will have a much more noticable effect.
 
Originally posted by: blazert40
So how was the drive?
Click to expand...

Not bad 🙂

I picked up a few used CDs before the drive and listened to Spanish and country stations from time to time (only types you can consistently pick up when driving across the southwest...note that I don't speak Spanish well nor normally listen to country 🙂)

I had planned to get a motel room to sleep through the especially hot parts of the day but the two Red Bulls I had on the road were having none of that 🙂

 
Originally posted by: akubi
what kind of a car do you drive? is it aerodynamic to begin with?
Click to expand...

1993 Honda Prelude with 136k miles.

Just eyeballing it, I'd guess it's at least above average in terms of aerodynamics as far as all cars go.

Extra weight was me (approx 160 lbs), and a duffel bag with about 40 pounds of stuff in it
 
fvck it, when it's hot out, it's all AC for me (too damn humid in the midwest to bother rolling windows down)
 
Am I the only one who gets a 404 on the first image?

Originally posted by: redly1
fvck it, when it's hot out, it's all AC for me (too damn humid in the midwest to bother rolling windows down)
Click to expand...

Yeah, seems like that season just arrived in full force today.
 
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