I just had a quick question. I always leave my AC on at 72ish degrees the whole time. My friend, however, rode in my car once and told me even if I lower my temperature to sixty's, the car will consume the same amount of fuel. Is this true? I have no idea on this one. I am just curious

 

Depends upon the outside temperature and how much the autoclimate system will need to use the AC compressor.

 

Exactly,

The only time an a/c compressor is actually conserving gas is when you are traveling down the freeway and a window open vs a/c compressor on = give the a/c compressor the winner here. Wind drag is the reason.

 

Actually, if anyone's watched the show on The Discovery Channel called "MythBusters", they actually tested this theory. They took two identical Ford Explorers, filled them up each with exactly 5 gallons of gas, and went 45MPH around a track. One had the AC on full and windows rolled up, and the other had AC off and all windows down.

The one with the windows down WON by a significant margin.

Now, considering that an Explorer on its own has lots of drag, an Explorer with windows open should have even more drag....and it still won. So, in theory, drag isn't so much a gas sucker as much as AC is.

Their original test with a computer hooked up to the Explorer's computer said AC on would be *marginally* better than windows down....but that machine based its analysis on airflow intake (which seems kinda weird).

 

I think that this is true at lower speeds such as 45 mph, but over about 65 I think the drag from having your windows down might waste more energy than the AC.

 

I would assume that the climate control system will cycle the compressor off when the interior temperature cools enough, rather than leaving it on a mixing in hot air, right?

(Though, If I leave my side cold and the passenger wants her side hot as hell, hypothetically speaking , then I guess it would have to mix hot air on her side while cooling my side.)

 

I think the compressor for the A/C stays on all the time unless you hit the button to turn it off. I am an A/C tech and am senstive to when the compressor comes on and off. Hit the button that says (i think) A/C off and see if you can feel and hear the compressor cycling. I don't believe that it cycles during normal driving, just mixes warm air from the heater to get the set temprature.

 

I've been tracking my mileage since I bought my car and I can see a decrease in MPG since I've started using the AC. I live in FL so AC usage probably started in early-mid may which is where my MPG starts falling off.

Date.....Miles Driven...Gallons... MPG
1/18/2005......387.8...13.9...27.89928058
1/31/2005.....370.9...12.999...28.53296407
2/11/2005.....407.9...14.36...28.39935947
2/25/2005.....365...12.869...28.36273215
3/4/2005.....184.7...6.537...28.25455102
3/17/2005.....364.3...12.604...28.90352269
3/29/2005.....384.7...13.317...28.88788766
4/15/2005.....356.3...12.501...28.50171986
4/27/2005.....390.3...13.43...29.06180194
5/11/2005.....386...13.452...28.6946179
5/24/2005.....376.8...13.554...27.79991147
6/7/2005.....369.9...13.541...27.31703715
6/21/2005.....369.9...13.408...27.58800716
7/4/2005.....365...13.134...27.79046749
7/14/2005.....345.3...12.556...27.50079643

 

I would tend to think the a/c would compromise the mileage more so than an open window, since its a hit to the engine performance whether you are going 1 mph or 100 mph.

Wind drag depends on the car's aero-design and is speed dependant for it to become high enough to equal the performance hit to that of running the a/c.

For me, I drive mostly light city traffic and my a/c is almost always on since I rarely go fast enough for the wind to cool off the interior. Secondly, the doors are too tall for my 5'8" frame so that I cannot hang my arm out the window. To that point, I keep the windows up.

 

The myth of wind drag applies to freeway speeds, not around town speeds, like someone said...it's mainly in the 65+mph range that you actually see the difference, it's not a whole lot.

 

lol thanks for the responses guys. But my question wasn't AC vs window rolled down

my actual question is.. if you set ur AC colder, does it use more gas than warmer AC(not really warm, but warm"er")?

 

Sorry to mis-read your original note. I would think lowering the temp would absolutely use more gas. Can't explain the mechanics of it but you simply can't get something for nothing.

 

I think the compressor would stay on longer initially to lower the temperature to the lower setting and then, perhaps, cycle on/off just a little more often to maintanin that temperature. Once the interior reaches a cold temperature it shouldn't take a lot more power to keep it there (unless you're opening doors or windows or not using recycled air). My

 

Actually, I did some testing today, idling and listening to the noises from the engine compartment carefully. The compressor definitely does not stay on the whole time. With the system in AUTO mode I distinctly noticed the compressor cycling on and off to keep the temperature at the set point. So, this means that setting it cooler than you really need would indeed result in greater energy usage (translating into a greater MPG hit), just like keeping the thermostat in your home set too low.

 

Sure it wasn't the radiator fan cycling??

 

Reasonably sure, though not 100%: I did some manual cycling of the AC first, followed by letting the the AUTO system do it, and it sounded exactly the same.

 

From what I've observed, the radiator fan is usually almost always on, but the compressor definitely does some cycling. I can feel the compressor going on and off on the highway pretty distictively. Fans going on or off changes nothing (or very little) to an engine's output since they are electrically driven.

Can't wait for all-electric-driven accessories!

 

The compressor does cycle on and off, you should be able to hear it. In my TL it makes a "hissssss" sound as it cycles on while in auto mode. The TSX is probably not as noticeable as it is newer.

On older cars with manual A/C, though, the compressor was on if the button was on and all you adjusted was fan speed, so no difference between low or high.