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

[GoSharks]

Originally posted by: b0mbrman
Just eyeballing it, I'd guess it's at least above average in terms of aerodynamics as far as all cars go.

look up drag coefficient numbers on google - i found values of around .34. for comparison, a 05 accord sedan is .30, and the coupe .29. a volvo s80 is .28. lower is better.

 

[Aharami]

hmm. was always under the assumption that driving at a constant speed with windows up and AC to 1st setting provides best gas mileage.

 

[sygyzy]

Originally posted by: StevenYoo
Mythbusters did a thing on this too.

same results: AC/windows down was less efficient

How can this both be true? Isn't the debate between fuel efficient of driving with your windows down (AC off) and windows up (AC on)? Who uses AC and windows down at the same time?

 

[PawNtheSandman]

Mythbusters concluded that they got 15 more miles per TANK with the windows rolled down compared to with the AC on.

 

[David Brent]

air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

 

[skypilot]

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

QFT

The A/C uses the same amount of engine power no matter which setting your cabin fan is on. The setting is just how fast some electrical fan pushes the readily available cold air into your face.

 

[Miramonti]

I'd like to have a .txt file with the raw data so I can run regressions in minitab to see if you know what the hell you're talking about. 😛

 

[shud]

Originally posted by: skypilot

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

QFT

The A/C uses the same amount of engine power no matter which setting your cabin fan is on. The setting is just how fast some electrical fan pushes the readily available cold air into your face.

Which is why the lowest setting always seems coolest.

 

[b0mbrman]

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)

Okay

What about the other 9 months of the year when it's just warm?

 

[StevenYoo]

Originally posted by: sygyzy

Originally posted by: StevenYoo
Mythbusters did a thing on this too.

same results: AC/windows down was less efficient

How can this both be true? Isn't the debate between fuel efficient of driving with your windows down (AC off) and windows up (AC on)? Who uses AC and windows down at the same time?

oops, typo on my part.

sorry about that. i meant AC/windows closed

 

[b0mbrman]

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

Fair enough. Just as long as it didn't hurt the numbers...

 

[b0mbrman]

Originally posted by: jjsole
I'd like to have a .txt file with the raw data so I can run regressions in minitab to see if you know what the hell you're talking about. 😛

Re: what?

Anyhow...here. Quote it and you should be able to paste it into excel w/the tabs still in...

Miles Gallons Avg temp °F MPG
205 7.881 78 26.01192742
179 6.308 79 28.37666455
352 10.692 92 32.9218107
249 8.501 98 29.29067169
296 10.947 92 27.03937152
314 11.507 84 27.2877379

 

[BurnItDwn]

I use windows when its between like 40 and 75 degrees ..
If it's more then 75, and sunny out ... I've got the AC on and set to 68.

 

[91TTZ]

Originally posted by: skypilot

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

QFT

The A/C uses the same amount of engine power no matter which setting your cabin fan is on. The setting is just how fast some electrical fan pushes the readily available cold air into your face.

This is completely incorrect. Your air conditioner does not use the same amount of engine power regardless of the setting. The AC compressor is attached to an electromagnetic clutch which cycles the compressor on and off. If you're not cooling the car much or it's cool outside, the duty cycle of the compressor will be much lower. If you're running it at full blast, the AC compressor will always be engaged. But it's incorrect to think that it uses the same amount of power all the time, since the compressor duty cycle varies with the setting.

Cliffs: When not much cooling is necessary, the AC compressor will disengage more often. Disengaged= not drawing power from engine.

 

[Raduque]

Originally posted by: Demon-Xanth
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.

Ain't that the truth. On my old '68 Oldsmobile, with AC on or off (In south texas heat) i got the same abysmal 13mpg. But man did I love that 355 Rocket 😀

 

[JulesMaximus]

Originally posted by: BCYL
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

Anyone really give a fvck? My comfort is far more important than saving a couple pennies on gasoline.

 

[91TTZ]

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

False.

 

[dullard]

Its fairly common sense.

Windows down is a minor aerodynamic drag, hurting fuel efficiency (but depends on speed).

AC on is a major engine drag, greatly hurting fuel efficiency (also takes power away when you need it so it also depends on speed).

Windows down is thus better in almost all conceivable cases. Mythbusters did one such test and yep, windows down was better.

 

[SynthDude2001]

Originally posted by: b0mbrman

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)

Okay

What about the other 9 months of the year when it's just warm?

I personally use the A/C then too (except during the months of December and January 😉)

 

[SynthDude2001]

Originally posted by: JulesMaximus

Originally posted by: BCYL
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

Anyone really give a fvck? My comfort is far more important than saving a couple pennies on gasoline.

QFT

 

[rh71]

yeah but having the windows down gives you a fuct up hairstyle afterward. If you wear a hat, it just gives you hat hair. 🙁

 

[LS20]

Originally posted by: 91TTZ



This is completely incorrect. Your air conditioner does not use the same amount of engine power regardless of the setting. The AC compressor is attached to an electromagnetic clutch which cycles the compressor on and off. If you're not cooling the car much or it's cool outside, the duty cycle of the compressor will be much lower. If you're running it at full blast, the AC compressor will always be engaged. But it's incorrect to think that it uses the same amount of power all the time, since the compressor duty cycle varies with the setting.

Cliffs: When not much cooling is necessary, the AC compressor will disengage more often. Disengaged= not drawing power from engine.

real truth. anybody with a small displacement motor and 5sp can attest... i can definately feel the power rob from the different settings...

ill have to test this on my own car (cD .35, 2.0 Liter, 220 mile trip from home<->school so i can keep speed and most things constant)

aside from 95*+ days, i actually prefer windows down... its a nicer sensation

 

[rh71]

don't tell me I'm the only one with hair...

 

[Cattlegod]

Originally posted by: rh71
don't tell me I'm the only one with hair...

only girls are allowed to complain about wind in their hair

 

[thomsbrain]

Originally posted by: 91TTZ

Originally posted by: skypilot

Originally posted by: David Brent
air conditioners draw the same power whether you have the temp all the way down or just down a little bit. the 9 degrees below outside temp is pointless, you're just wasting a lot of cooling power.

QFT

The A/C uses the same amount of engine power no matter which setting your cabin fan is on. The setting is just how fast some electrical fan pushes the readily available cold air into your face.

This is completely incorrect. Your air conditioner does not use the same amount of engine power regardless of the setting. The AC compressor is attached to an electromagnetic clutch which cycles the compressor on and off. If you're not cooling the car much or it's cool outside, the duty cycle of the compressor will be much lower. If you're running it at full blast, the AC compressor will always be engaged. But it's incorrect to think that it uses the same amount of power all the time, since the compressor duty cycle varies with the setting.

Cliffs: When not much cooling is necessary, the AC compressor will disengage more often. Disengaged= not drawing power from engine.

QFT.

In my 1991 Accord, any fan setting over 3 runs the compressor all the time, but lower settings run it progressively less often. At "1" it only kicks on every once in a while.