Is there any way to esimate it’s cost?
It must use more gas, but is it measurable?
Am I paying 1 an hour or a month?
If you are driving the highway you are probably breaking even using the Ac vs. having windows open. It has to do with extra drag created by having an open window.
Cecil touches on the subject: What gets the better gas mileage: windows up, AC on, or windows down, AC off?
And using Unca Cece’s admittedly limited research to answer the OP:
That’s a difference of .6 MPG. Now the math:
34.4 MPG = .029 gallons to the mile, or .01744 gallons for .6 miles
I paid $1.68/gallon last night when I filled up, so it costs me about 5 cents per mile at that gas mileage to use the A/C, or about 3 cents per .6 miles
If you average 1000 miles per month (like I do), that comes out to $30 per month savings. Of course this is all dependent on if you might have used the A/C at all on a particular day versus having the windows rolled down.
And since I misread my own quote, that should be “…it costs me about 5 cents per mile at that gas mileage to not use the A/C and have the windows rolled down…”
So you save $30 per month by using the A/C.
Cecil’s article is wholly unscientific and counter to everything I have experienced. Ditto others I have talked to. It’s quite unfortunate it is still be cited.
Windows down costs me a little mileage. (Around 3%.) AC is a big loss. (Around 20%. So I never used it and now it doesn’t work at all anymore. Not a problem.)
Only by doing careful tests on your vehicle under your driving conditions can you hope to estimate how much having the AC on costs you. “YMMV” is never truer than in this case.
Drove a nearly 240-mile round trip twice in the Mustang. Once in the heat of Summer, air conditioning on - the second time in November, no A/C, and windows rolled up.
Driving style: Cruise control on 85 mph. Straight highway in Western Kansas (I-70).
Each time, tire pressure was inflated before the trip to the rated maximum (44 psig).
Mileage the first time (Summer, A/C on): 26 mpg.
Mileage the second time (Fall, A/C off, windows closed): 25 mpg.
Why the difference, even with so many factors being the same? Possibly due to air and engine temperature, different gasoline, different driving style, different head/tail wind…there are so many variables it’s very hard to tell. But whatever the case, there was not a dramatic difference.
I did some searchs and found nothing that seemed well supported on this. There’s a lot of suggestions that not using A/C saves gas but not addressing the issue of highway drag increase vs. A/C. I remember a few years ago the manufacturers were saying, because of improved aerodynamics, windows up A/C on was the way to go, but they want to sell A/C, of course.
The other factors such as air temperature, head winds, driving style, traffic etc. are much more important. I know I seem to see significant losses when temps get above 80 def F or below 40 deg F. High air temps cause a reduction in gas mileage that may be mistaken for an A/C effect. Engine size matters too. Very small engines suffer more from the A/C load.
Driving on the highways is so much more comfortable with the windows up I use the A/C anytime I feel like it.
P.S. Don’t forget your A/C may run in the winter when you use the defroster.
[quote]
I remember a few years ago the manufacturers were saying, because of improved aerodynamics, windows up A/C on was the way to go, but they want to sell A/C, of course.
Can you even buy a new car without it these days? Special order perhaps?
Well, let’s think this through a little bit. Let’s look at something with two different engines available: A typical Ford Focus 4-banger. One engine is the 2L Zetec with a peak 130 hp. Another is the SPI with 110 hp. Both weigh 2600 pounds.
Regardless of the horsepower at a constant speed on the same road with all of the same conditions, both cars will consume the same horsepower, i.e., it takes the same amount of work to make them move. Yeah, the higher hp engine will get you up to speed faster (really, this is torque), but once there there’s no difference. Of course different engines will be more or less efficient so they may use more or less gas, but bear with me. Let’s say it requires 15 horsepower to keep at 45 mph. Yeah, that’s low. I’m not in the mood to do all the maths, but I’m probably not far off.
Now factor in the air conditioner. I have no idea what an air condition consumes in hp. Let’s say it’s one hp. That’s an additional 6.7% load on your engine!
Now consider a larger car with a larger motor. Take Anthracite’s Mustang. Let’s say it’s the 260hp version. The car will get up and move, and weighing in at 3300 pounds. It’s 127% percent of the Focus’ weight. Let’s say it takes 127% of the Focus power to keep it running at 45mph: it’s using 19 horsepower. This doesn’t account for different aerodynamics and engine efficiencies, but we’re close enough, and remember we’re dependant on my original 15hp being close enough to be accurate. The air conditioner, though, doesn’t have to be stronger – it’ll consume 1hp. That’s a 5% extra load on the engine. That’s versus the 6.7% on the Focus. A noticeable difference.
Okay, I can’t say whether air conditioning or rolling down the windows is more efficient at this point, but I can say why an econobox suffers more than a good-sized car when the A/C is in use.
<side note>
Why is it only people that drive econoboxes worry about their mileage all the time?
</side note>
So the next logic step is to actually measure the HP your car consumes with the windows up versus the windows down, and factor in how many HP your A/C unit consumes. For all I know this could be on the A/C unit somewhere, in the owners manual, or your local dealer will know.
I’ll have to dig out my books at work on Monday for this – been a long time since I’ve had to do anything with horsepower. But, you should be able to measure you hp by getting your car to a set speed, coasting to a set speed, and timing how long it takes. Since coasting decelleration isn’t linear, though, you’ll need to coast from, say, 45mph to 40mph, otherwise you’ll skew the results. Repeat this with windows both up and down, and you’ll have an idea how much hp the windows take vs. up. Factor in the A/C hp, and we’ll have an answer. Of course I won’t get back to work 'til Monday, so, anywant want to offer up a quick and dirty formula?
Just a couple of additions to your summary:
Internal frictional hp losses of the engines may be different, but perhaps not by much.
FYI, reputedly (and I do stress “reputedly”, because I cannot find my spec sheet) the aircon on my car (98 GT) requires between 4 to 7 horsepower, depending on the engine speed.
Well, another thing to consider is I have a markedly larger rolling resistance with my 245-series 17’s. And I would say, anecdotally speaking, that my aircon is quite a bit stronger. It chills the car like a freezer even on a 100 F day, in just a minute of running. I typically have to run with the fan on lowest setting, or else never wear a skirt.
Well, that’s likely one of the deciding points for their purchase. And it’s a way they try to optimize things to save money. Me, I just need power - and when I get my 2004 Cobra, I will finally start to approach the power I truly need. Gas mileage is unimportant to me because relative to my salary, gas at $1 a gallon or $5 a gallon is really not something that bothers me. Even when I drive in the UK (with petrol at nearly $4.50 per US gallon) I really don’t concern myself with the cost. It sure wasn’t that way when I was at university and delivering pizzas to pay for school…
Una