A/C Impact on MPG
May 22, 2011 1:16 PM 
Subscribe
SubscribeDepends on what speeds you are driving at. Generally anything under around 45mph you are better with the windows open. Once you get above that general speed, put the windows up and turn the A/C on for better fuel economy due to the air drag.
I beg to differ. Can honestly hand on heart say that I notice no difference what so ever with my A/C on, so much so I now leave it on 24/7 even in the winter. I'd just stick to what your more comfortable with, either with windows down, or AC on.
When the A/C compressor gets turned on, it does create an additional workload for the engine. When the A/C is not on, the clutch on the A/C pulley disengages and it "freewheels",so to speak. With a small engine, the workload caused by the A/C operation is much more noticeable than in a large-engined car. And, fuel economy on that small car will suffer somewhat.
Having the windows open used to be a major factor, but the streamlined design of MANY cars out there, big and small,has lessened drag coefficients considerably, and open windows don't really make much difference.IMHO.
Myth Busters did an episode on this. I just cant remember exactly what the out come was.
Many 2010 and newer Audi models have a feature called the Efficiency Program. It show fuel-saving tips on the onboard display. It also reports consumption estimates in gal/hour (L/hour) for various devices, including the A/C. So, at least in Audi's opinion, the effect of the A/C is not negligible.
You can see a brief video of the Efficiency Program here:
http://www.audi.ca/ca/brand/en/exp/Efficiency/efficiency_technologies/assistance_systems/onboard_computer.html
While I agree that the newer A/C systems aren't as big of a horsepower pig as those of decades past, I can tell you from experience that on my wife's 2005 Mazda MPV (minivan) and my 2000 Acura Integra, I notice a drop in mileage with the A/C running. My last car took approximately 10 hp to run the A/C which worked out to about 1 gallon per hour of gasoline, the Acura now burns about an extra 1/3 to 1/2 gallon per hour with the A/C on by my mileage tracking, the Mazda uses more like 2/3 to 3/4 of a gallon per hour, larger interior volume to cool, but has a lower relative MPG impact due to the average MPG being lower to begin with so an increase in fuel consumption doesn't show up as glaringly as it does with a car that burns about 2 gallons per hour at 65 MPH when it increases to 2.3 or 2.5 gph the mileage drops from 33ish to 27ish a loss of almost 20% as opposed to burning 3 to 3.5 gph at 65 and increasing to 3.6 or 4.2 for a mileage drop from about 20 mpg down to 17ish for a drop of more like 15%. For some reason that doesn't seem to hurt quite as much.
I beleive Mythbusters concluded the same thing that BigDog stated. At slower speeds windows down helps, higher speeds, the parasitic drag caused by windows down is greater than that caused by the AC.
If you think about it, even with the aerodynamics of newer cars, with the windows down, a lot of wind is blowing around in your car. If minimal aerodynamic losses were occuring, you should be able to drive with windows down and not notice any air coming in.
To complete was everybody is saying about the particular Mythbusters episode, they concluded with driving under 50mph is more efficient with the windows down and driving over 50mph is more efficient with the windows up and A/C on because of the drag caused by the windows when they're down. It's kinda like a truck's tailgate.
Nevertheless, I don't have A/C on my car (bone stock) and I like it that way. I noticed on the 2010 Mazda 3 that it idles at 850rpm with A/C on and approximately 750rpm when it's off.
ScangaugeII shows a drop of 3-4 MPG with my A/C on. That's roughly a 10% drop in fuel economy. This is reflected in my generally better MPG figures during the Winter months.
Everyone also needs to keep in mind, not every vehicle has the same results...
Appreciate everyone's comments on this thread...very interesting and helpful!
My '97 Ford Escort will get better mileage without the a/c in constant speed highway driving. I don't know what it does on shorter low speed trips, because I normally use this car for longer highway trips. On the highway with c/c set at 55 mph and a/c on I usually get 40-42 mpg the same conditions without the a/c on I can get 43-49 mpg. My best mileage ever on this car was a cool summer day when I didn't need the a/c and drove with the c/c set at 55 mph with the windows down, I achieved 49.85 mpg.
This is one of many reasons it is good to have fuel consumption instrumentation. If your car didn't come with any I would suggest a scangauge, ultragauge or something similar. Otherwise you are driving blind when it comes to fuel efficiency.
I doubt if my '05 Odyssey is unique in that the idle speed, as far as I can tell, is identical whether it's in neutral or drive, and whether the A/C is on or not. It's electronically regulated. Of course the lower the load on the engine the less gas it should use at idle. At shorter stop lights I turn off the A/C and put it in neutral. With climate control in recirculate, it stays cool enough for a while on many days before I must turn A/C back on. At long red lights, I'll turn off the A/C and the engine. At cruising speeds between 45 and 70 or so, vehicles need little power to keep moving (this car runs on 3 cylinders), especially with the windows closed, so running the A/C uses very little gas.
Have noticed over several summers that when it is on all the time, the penalty is about 1.5 MPG loss. Although that is less than the winter fuel penalty.
I have an old Series 3 Land Rover. It doesn't have A/C. It has a flap on the front to let in air :-)
I'm really not sure whether the difference is as much as bandied about, particularly with diesel vehicles and modern climate control systems. I have tried hard to measure a difference, and it is hidden by all the other variables.
One problem is that climate control systems are set to a temperature, and so the air-con might well not be engaged a lot of the time. In addition, diesel engines generally have a lot of torque, so the load is not as noticeable as with small petrol engines. Furthermore, with modern HDi engines, there is no fuel consumption when running with no throttle. Yet the aircon will still turn if engaged. Thus, going downhill in gear, the aircon has no effect on fuel economy at all.
Another confounder is that if the external temperature is warmer, the engine will reach its operating temperature a lot sooner, and so run more efficiently. My engine idles at the same speed whether the aircon is on or not. The ambient temperature and the speed you are driving at also has an impact. The aircon also has to dissipate heat under the bonnet. How efficiently it does this will have an effect upon how efficiently it can cool the interior. Travelling at speed will maximise the airflow over the aircon radiator. A lower ambient temperature will increase the differential between the radiator and the airflow, again improving the cooling of the radiator.
Perhaps the best way to determine how much of an impact it has would be to measure the change in 0-60mph times with aircon engaged and off. If it has that much effect, it would change the performance of the vehicle. On an old 1.1l Suzuki Swift, it used to slow down if you were on the motorway and engaged the aircon.
Travelling around in traffic in temperatures of 45°C will result in the aircon having a greater effect on MPG compared with driving around in temperatures of 25° on the motorway.
In order for me to find traffic with temperatures of 45 Centigrade, I have to go to Canada or Mexico.
Where in CANADA are you going to find temps of 45 degrees!!??!!?
I doubt that any part of Canada gets over 40!
Mexico however, no problem. lol
In my old 4.7L HO Jeep the A/C compressor was such a small load on the V8 you never noticed it. Now in my 1.8L Integra I can feel and hear each time the compressor clutch locks. Do a tank with A/C and one without keeping other variables the same and see how your car handles it.
In a cross-country road trip in my 2006 Jeep Grand Cherokee (the trip was in July 2008), driving across the northern states and into Canada, the temps were cooler and we didn't need to use A/C. On the return trip, we drove down the East Coast and across the southern states where it was very hot and humid, so we were using A/C all the time. When I calculated my fuel economy, there was a noticeable 2-3 MPG drop when the A/C was on vice when it was off.
It would be a nice addition to the website if there was an option to record if we were using A/C or not.
You could use notes or tags to record your A/C use.
My Civic Hybrid used to drop 10 - 15% with air conditioning - 4 - 6 miles per gallon. THere was even a warning in the owners manual about it.