A/C Impact on MPG
 

Today's car A/C systems are probably more efficient than those days of yore (60s/70s/80s). I was just wondering if turning off the A/C for a 2010 car would result in noticeable improvement in fuel economy.

Depends 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:

https://www.audi.ca/ca/brand/en/exp/E..._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.