As the summer approaches and warmer weather is here, many will wonder about the effect of air conditioning on mileage.

I'm no expert, but I believe that for 'normal' cars, driving with the windows open reduces mileage (due to the drag) about the same as using the AC. The AC's are a lot more efficient than they used to be. So there is no benefit in sweating...unless you want to lose weight.

As far as hybrids are concerned (I am not positive about this, but believe I read it somewhere, so if I am wrong, please excuse me) the AC runs off electric power, not tied into the engine. If I understand it correctly (and I probably don't), a hybrid driving with full battery shouldn't have any mileage effect, since there is ample battery charge. Of course if the battery is low, then the ICE will need to run to recharge the battery. Correct me if I am wrong.

Other great tools:

• Toyota Online's iGuide
• 2007 Online Owner's Manual
• 2008 Owner's Manual PDF (download link)

I think the AC does have an effect on fuel efficiency. The compressor uses a lot of energy. The fact that it is electric just means the energy has to go from the gas engine, through the generator to the battery, and finally to the compressor. That has got to be less efficient than going straight from the engine to the compressor. The advantage, of course, is that the AC can run while the engine is not.

I have an anecdote to support this theory: I live in south Florida. We had a heat wave this spring. It lasted about 6 weeks. Temperatures were running mid-90's during the day, and 80's at night. (Thats at least 10 degrees warmer than normal for April-May.) And, humidity was much lower than normal, though I don't know the number. I got two tanks of miserable gas mileage during that period -- at least 3 mpg below what I had been getting. The AC was running on high almost all the time. The heat wave finally broke about week ago, and my mpg's seem to be just about back to normal since then.

Steve-

I just recently heard on an MSNBC program(maybe CNN) that running A/C costs you about 1 mpg. I guess that was on low, not high. So 3 mpg on high and maybe 1 mpg on 'ECO' I would say is fair. thoughts?

No question in my mind that AC hurts mileage. My TCH gets optimal mileage when the outdoor temperature is about 70 degrees with windows open.

In very hot weather you do have two competing factors since the battery capacity is reduced when they are hot (by the computer) to preserve them. Running the AC in hot weather helps cool the batteries.

The benefits of the TCH are definitely reduced at either temperature extreme. I wonder it the Lithium Ion batteries will be any better in this regard?

My best mileage occurs in the spring and early fall when I am not using the climate control system. Windows open.

When I have to run my A/C even in ECO mode, the mileage drops. However, the winter months are by far and away the worst for mileage on my car.

ac runs entirely on electric. the difference in mpg can be attributed to the higher ambient temperature, which is the reason you are using the ac!

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On engine-driven A/C, the compressor needs to be designed in a way that provides max cooling throughout a large range of engine RPMs (1200 - 6250 on Camry). To cool well at low RPM, the compressor must be large and powerful, which reduces efficiency. Also, the compressor RPM cannot be adjusted based on A/C load which reduces efficiency.

The electric compressor does not rely on the engine RPM at all, so they can optimize it's size and performance for specific RPMs based on A/C load. I don't know exact figures, but the electric A/C is measurably more efficient, even with losses due to "Engine->generator->compressor motor" energy conversion.

I don't think I have ever not used the AC. This is south Florida. Around here, people run the AC while driving convertibles with the top down in the winter.

The difference between the normal amount of AC and the "constant high" AC was about 3 mpg. I would guess the difference is more like 1 mpg on 'ECO' and 4 or 5 mpgs on 'high'.

Here is my 2 cents worth. On 85 to 90 degrees weather the ac pulls the mpg down about 1 mpg. At 90 to 95 degrees I loose about 1 1/4. With the temp 95 to 100 about 1 1/2 mpg. Over 100 to 105 degrees 2 or maybe 3 mpg.

This also would depend on headwind, crosswind and the outside humidity.

On a hot day it's best to set to fan to 3 bars when you first get in the car. You can always reset the ac once the car interior cools off after 10 to 15 minutes. Adjust the ac temp to 78/79 in the dryer areas and about 73 to 76 in humid areas. After the car has cooled some then try setting the fan to 2 bar graphs. I found many times after dark on a hot evening one bar on the fan works ok.

I tired the auto setting an on a hot day the fan speed runs way up to about 6 to 8 bars. I figure a lower fan speed would save some gas. I found temporarly rolling the windows down and driving about 200 to 300 feet really helps clean out the hot air.

...wow! Nice researching, you take this pretty seriously, huh?
I was raised with "270 Airconditioning" (roll down the windows and do 70 miles per hour), so to this day, I rarely use AC, I prefer to drive with open windows (yes, I know it causes drag and kills MPG, so if the world runs out of oil you'll all know who is to blame). If I am driving with people, my frugal ways (assuming using AC=$$) are hidden and I splurge.
If one can save 3 MPG for 12 months (I doubt we'll see 100 to 105 degrees every day for a year) assuming 15,000 miles per year is approximately 26 gallons a year, or $100 per year. That's a lot of money, but that's the equivalent of either

• one venti mocha latte with two pumps every two weeks
• or one ticket to a MLB game with parking and a beer (or two),
• two months of high speed internet
• dinner for two with a few glasses of wine
• 27 minutes on one of those naughty "976" telephone numbers...don't ask me why I know that one!
  

I guess my point is...drive the way you feel comfortable, if you're hot, put on the AC, if you're cold, put on the heat. You single handledly wont save the earth, nor cause it's ruin.
Enjoy life...it's really short!

Other great tools:

• Toyota Online's iGuide
• 2007 Online Owner's Manual
• 2008 Owner's Manual PDF (download link)