how much money can I save by turning off air-conditioning when I drive?

September 18, 2007 1:25 PM

how much money can I save by turning off air-conditioning when I drive?

I know that turning on air-conditioning increases fuel consumption. How much money can I save per month if I go from using air-conditioning every time to not using it at all?

I drive a small 2002 car with 1.5L engine and manual transmission, and fill up my tank with 35L of gasoline twice a month. This costs me about $70 per month right now (Canadian dollars). My driving is approximately 40minutes a day, five days a week in the city.

If you could explain, how are the savings calculated, that would be great too!

posted by esolo to travel & transportation (21 comments total)

Is this for a report/science project, or just a matter of curiosity?

Because, there are several dozen measures out there, ranging from 4% savings to 20% savings, depending on A/C, type of car, how often you run it, type of climate, etc.

All in all, for your $70/month, I doubt you'll save more than $10, but that's unscientific, I acknowledge.
posted by The Giant Squid at 1:29 PM on September 18, 2007

I bet this easiest way to determine this is just to do the experiment. Track your gas usage for a few months using the A/C and then turn it off for a couple months and track your gas usage again. It might be hard to measure, though - it would surprise me if the change in gas usage is no bigger than variations in gas usage due to traffic, etc.
posted by pombe at 1:32 PM on September 18, 2007

You can save a lot if you just remember to turn the AC off ten or fifteen minutes before you get to your destination.

Why? Because that gives a while for the outside air coming through the system to evaporate the condensation from inside your vent system (you know that water that's trickling out under your car when you stop, after the AC's been running? That's condensed water from the cooling coils, but they don't really dry off just from gravity draining without any air moving).

The cost of pulling an AC system completely apart to clean it out with enough chlorine bleach to take away that mildewed smell you eventually get otherwise is in the hundreds of dollars, and sometimes it doesn't work.

----
On point, try it and find out. Drive the same trip with AC on and off, consistently, for a few days, filling the tank and checking. We save about 10% on gas not running the AC, based on a 120-mile mostly flat freeway trip done several times with and without.
posted by hank at 1:33 PM on September 18, 2007 [1 favorite]

If it means driving with your windows open instead, nothing. I remember reading that the two are on par when it comes to fuel consumption, due to the increased drag caused by the open windows. The Mythbusters find them to be pretty close.
posted by craven_morhead at 1:33 PM on September 18, 2007 [2 favorites]

Depending on the car you can lose 2-5mpg. On average that 3.5. (which might be high for modern cars but lets run with it.)

Lets assume your car get 23 mpg on average. You burn 9.24 gallons a month. That means you drive 212 miles a month.

Now lets assume you leave the air on the whole time and you get 22.5 mpg. With this setup you'll burn 9.42 gallons per month. Thats .22 gallons more. Almost one liter.


So, whatever a liter costs.
posted by damn dirty ape at 1:39 PM on September 18, 2007

I've heard that driving with the windows down instead of using the a/c is as if not more inefficient in that it creates way more drag.
posted by zennoshinjou at 1:40 PM on September 18, 2007

Didn't Mythbusters revisit this myth? I believe there was something about a certain speed being the breakpoint..

asavage where are you!!?! :)
posted by radioamy at 1:45 PM on September 18, 2007

Whoops. I should have wrote you get 19.5 mpg with it on. Thus you'll burn 10.87. SO you'll burn an extra 1.63 gallons per month or 6.17 liters. So whatever 6 liters costs you.

That's on the high side. Some sources say AC only effects the mpg by 5% thus closer to my original estimate of 1 liter. In real life I'd assume 3-4 liters burnt.

There is also a mild loss of mpg by driving with the windows up but isnt close to what ac uses. The mythbusers episode is pretty flawed.
posted by damn dirty ape at 1:45 PM on September 18, 2007

I've heard that driving with the windows down instead of using the a/c is as if not more inefficient in that it creates way more drag.

Seconding zennoshinjou, I heard the same thing on Car Talk several months ago.

(Why yes, I do consider Click and Clack to be the ultimate in automobile maintenance gurus.)
posted by mewithoutyou at 1:46 PM on September 18, 2007

I misread your question. You fill up 2x a month. So if you burn an extra 4-6 liters then its 8-12 liters per month. Looks like gas floats around one dollar a liter there so my final answer is: 8 - 12 dollars. One average: 10 bucks.
posted by damn dirty ape at 1:54 PM on September 18, 2007

You can probably save a lot more money by driving conservatively than you can by not using the AC.

I was curious about this recently, so I played with the A/C on full blast as cold as it would go vs. no A/C at all, windows up in a 2007 I4 CR-V with the cruise set at 75. The difference was about 1.5 mpg, if I recall. About 24.5 mpg v. 26 mpg. That is was less than the differences I have found generally when varying the way that I drive. Generally, I have found that to have about 3-4 mpg of impact in city-traffic-type highway driving. Driving really fast and flooring it a lot leads to abotu 21-22 mpg, whereas really being careful and trying to make that fuel efficiency meter be all the way to the good makes it about 24-25 mpg, for traffic moving around 50 - 70 mph.

No idea if the running digital MPG gauge in the CR-V is that accurate, though.
posted by iknowizbirfmark at 2:13 PM on September 18, 2007

I remember reading that on a modern car there was aprox. 2% difference between the increased gas usage caused by increased drag on the car by windows being down and the gas consumed by AC with windows up. It's been a while so I don't remember the source, but I've made a note not to use both at the same time because of that.

Wikipedia article on the MythBusters test...
posted by TheDukeofLancaster at 2:37 PM on September 18, 2007

Before moving to be closer to work, I had a 80km (each way) daily commute, that was 90% done on 90km/h highway. So it was optimal for me to see just how good of mileage I could get from my 2004 toyota echo.

I'd get betwen 19-19.8 km/l until I started using the AC, when I started getting 18-18.5 km/l .

Part of that could be that the engine doesn't operate as well in the heat, but I'll just assume that it was all AC. That's a bit over 5% more gas to run the AC.

However, if you're going to run with your windows down instead, as mentioned mythbusters showed otherwise. Specifically, in their car, up to 45mph it made sense to roll the windows down. After 45, it was more fuel efficient to use the AC. Different cars will vary on the sweat spot, but if you're driving on american highways (read: 65mph+) it makes sense to hit the AC if you can't deal with just the vent.
posted by nobeagle at 3:10 PM on September 18, 2007

Just a note about the windows down vs. A/C. I asked that question a while ago and got some great answers.
posted by chrisroberts at 3:29 PM on September 18, 2007

A friend of mine did this experiment with a 2.8L BMW (1998) and found next to no difference. Proven by the fact that he still uses the A/C and is a tight arsed Yorkshireman (self proclaimed and celebrated).

Bigger savings to stop you worrying about teeny savings from the A/C:

1: Time your journeys better. Leaving 10 minutes earlier can shorten your journey time considerably (as can 10 minutes later) and mean you spend less time in the car for just being more aware of traffic swells.

2: Better awareness of traffic and congestion - knowing that if there is a large tailback at a particular exit allows you to hit a side road and maintain a better average and avoid stop-go traffic is a big winner.

3: Linked to the above and better for longer trips mostly - Cruise control. It encourages you to concentrate on cruising at (say) 110km/hr rather than being tempted to get in the 100-120km/hr concertina that lanes in Canada seem to do. You get there at the same time, but the constant speed saves loads on fuel usage.

I use the above (Toronto commute of 4o minutes average) and have a 5L V8. Nothing like an old shit box for making the savings brutally apparent...
posted by Brockles at 4:20 PM on September 18, 2007

You can break even easily. That is, you can make it no more expensive to run the AC than not to.

You should acknowledge the drain on engine power from the AC, and accept the consequential impact in performance. If, however, you adjust and open up the throttle farther so that you shoot ahead just as quick as you would without AC, you'll burn more total fuel than you would without AC on. Go at a slightly slower speed on the highway, accelerate less (in both directions).

On the other hand, if you have a giant engine, you probably won't notice any difference with the AC on or off. My buddy's Vette undoubtedly feels precisely the same and guzzles just as much gas regardless of how high up he turns the AC.
posted by Netzapper at 4:31 PM on September 18, 2007

You sound to me like you do a lot of interstate driving, or at least highway driving. If you're doing this right, even in your little 1.5L car, you shouldn't be pulling more than ~2200-2500 RPM's at this point. Late model cars' AC systems don't usually add more than 200-300 rpm's to handle the compressor AT MOST, and really when cruising you're in a low torque mode anyway, so you're as close to a 0 effect as possible.

Now, driving in town, when you should be stopping and going and idling around 800 rpm's or so, those 300 or so turns will make a huge difference efficiency wise, not to mention putting incredible wear on your system since it won't cool itself nearly as well w/o air moving over itself.

Now, with that said, when I'm just *warm* in my car, I'll often turn the fan on the coldest setting w/o actually turning on the AC, and that usually does the trick.

You could also get some mirrored tint for your windows, make sure your tires are aired up, oil changed, plugs and wires fresh at least every 30k miles, avoid racing stoplight to stoplight and immediately trying to regain speed after slowing, and you'll see a much more significant improvement in mileage over some AC use.

One more note---where I am, which is in the mountains, midsize 4 bangers often do poorly on gas consumption compared to 6 cylinders, because the engines have to tax themselves so hard to climb up the hills, which is sort of a weird phenomenon to me.
posted by TomMelee at 6:16 PM on September 18, 2007

I just did this informal experiment.

Using the a/c a bunch: ~28 mpg

Using the a/c not so much: ~33 mpg

YMMV...ha ha ha ha ha!
posted by spikeleemajortomdickandharryconnickjrmints at 6:49 PM on September 18, 2007

When I tested this not too long ago (1995 BMW M3) on flat highway with cruise set at 68, I got 31.2 mpg with the A/C off and 30.7 with it on.
posted by ikkyu2 at 6:55 PM on September 18, 2007

For what it's worth, AC has a very minimal effect on mileage in my Maxima. My record gas mileage was on a tank where I ran the AC for probably 300 out of the 500 miles I drove. (Got 31.4mpg)

For me, the biggest three factors to improved gas mileage, in order of significance are:

• Limit top speed to 60mph. Air resistance grows proportionally to the square of your speed, so going from 60 to 75 (a 25% increase in speed) increases drag by 56%. Most cars are at their peak efficiency between 50 and 60mph.
• Inflate tires properly. Mine are at 34psi all around, which is 2psi above the recommended pressure.
• Accelerate slowly, whenever possible.

I've practice other techniques, like shutting down the engine and coasting on long hills (dangerous and possibly illegal, so take care). I only do this if I can cover at least half a mile before I will restart the engine, typically it's on hills longer than 1 mile. I keep the windows closed if I'm going over 50mph, and use AC if it's hot. Obviously no extended idling, at traffic lights or drive thrus (when the engine is cold, I don't shut it down, it's kinder to let it get fully warm before doing any shut down/restart cycles, and the engine is more efficient when it's at operating temperature).

The typical owner of my car (2001 Maxima, 5 speed) gets around 23mpg according to fueleconomy.gov. I regularly get 29 - 31, and the biggest impacts were the 3 bullets above.
posted by knave at 10:53 PM on September 18, 2007

For my '97 Accord, in the winter when I'm driving with neither A/C nor the windows down, I average 31.5 mpg. In the summer, with A/C I average 30.6 mpg. With the windows down, 29.8 mpg. This is all for mixed city/highway driving.
(Measurements averaged over a month each, all taken last year and with no scientific rigor whatsoever.)
posted by solotoro at 11:02 PM on September 18, 2007