I had to drive a car w/o air conditioning in florida over 2 summers (it was an 83 and the we cant get the freon for it cuz its illegal). I now drive a focus and have the a/c on most of the time, havent noticed a diff in gas mileage. Mythbusters tested this with 2 expeditions, 1 with a/c on and one with all windows open at 45 or 50 mph. The one with the a/c on died first even though everyone thought the one with windows open would have first because of the extra drag from the open windows.

 

I always ride windows down.. A/C robs to much power from the Engine when its on. But really it doesnt have much effect with steady highway driving. Fuel economy will be the same on or off.

 

What he said, Using AC puts more strain on the engine thus losing power, and robbing gas mileage, and having windows open is much nicer imo...

 

Fresh air it is, unless I'm talking on the phone, thanks guys!

 

i had read a test done on this and it turned out to be better mpg with the a/c on actually due to the drag from the windows being opens , you dont have to hqave the a/c on full blast bc the climate in the cabin will eventually be to what your comfortable with and with just need a low power to keep it there along with the car moving at a steady speed....

 

i had read a test done on this and it turned out to be better mpg with the a/c on actually due to the drag from the windows being opens , you dont have to hqave the a/c on full blast bc the climate in the cabin will eventually be to what your comfortable with and with just need a low power to keep it there along with the car moving at a steady speed....

if the a/c is on it does not matter how high it is. the a/c is only on and off. the fan speed does not affect gas milage or power. so if you use the a/c dont be shy to crank it up.

 

"if the a/c is on it does not matter how high it is."

Not true at all. The compressor cycle time is altered by how fast the heat is taken out of the heat exchanger in the HVAC system, and the faster the fan speed, the faster the heat is taken out. The more time the compressor is on, the more negative impact on fuel economy.

 

there really isnt much "drag" when driving with the windows down, especially at anything close to legal speeds.

basically, when I'm in town, driving less than 50 mph, windows are down, if I'm on the highway, it's windows up, A/C on.

 

^^^ i meant highway miles , not around town....

 

i always drive with my windows down, but thats cause i love the feel of the wind in my face.

i have heard that at highway speeds, the mpg pretty much evens out. i think you would see more differance in stop and go driving, as others have said.

 

Here's a couple of bit of real data, vs. opinion:

"* Several sources claim that closing windows and using a car's air conditioner will provide better fuel economy at freeway speeds than leaving the windows open without the air conditioner. However, FSEC tests showed this in not the case. In repeated evaluation at 65 miles per hour, our test car experienced 11% better fuel efficiency with no A/C and the windows open than using the air conditioner. We also found that closing windows at freeway speeds improved fuel efficiency by 2-3%.

* Other tests done more recently by Edmunds.com showed similar results to those from FSEC: open windows without air conditioning at freeway speeds was, in fact, more efficient than closed windows with air conditioning: http://money.cnn.edmunds.com/ownership/driving/articles/106842/article.html
"

 

what do you guys call hot......it gets to 150 where i live?!?


70 is the best weather that we have

 

^^^ Ditto, 70 is a nice middle of December temp here.^^^

 

i go for both, it depends on the situation

edit* if i were worried about gas mileage i wouldnt have spent so much money on performance parts. thats right, this little beast is my "race car". at least thats what i tell the ladies!

 

"if the a/c is on it does not matter how high it is."

Not true at all. The compressor cycle time is altered by how fast the heat is taken out of the heat exchanger in the HVAC system, and the faster the fan speed, the faster the heat is taken out. The more time the compressor is on, the more negative impact on fuel economy.

Ok, Thanks, i guess i worded that poorly. The compresser is indeed only on or off though. yes?

 

that show MYTH BUSTERS on discovery channel tested this....and their results showed almost no difference with either a/c on or windows down...they ran the same car with a full tank of gas around a track at the same speed ...once with the a/c on, and the 2nd time with the windows down and no a/c and they ran out of gas after the same amount of laps.......i do both myself...depends on the damn new orleans humidity....

 

Ok, Thanks, i guess i worded that poorly. The compresser is indeed only on or off though. yes?

Yep, and the AC will cycle more frequently the lower the temp is set. Our vehicles have a little bonus though- thanks to modern engineering. Our AC compressor is limited to the # of rpm's it will turn, and will cut off at WOT. The first bonus is longer compressor life, the second bonus is that it won't rob power if you need it for acceleration. I imagine that there is some sort of rpm limiting at higher non-WOT engine speeds also which means less drag on the engine.

I am not sure if the recirculation setting (AC Max) affects compressor cycling like in older US cars. If not, then I'd definately keep the system in recirc mode while the AC is on.

A good common sense idea that hasn't been listed here: If you prefer to have your AC on while driving, leave it off for the first 2 minutes of your trip and roll down all the windows to release trapped hot air.

 

"The compresser is indeed only on or off though. yes?"

Yes, but the percent of time it is "turned on" depends upon how "high" you have the A/C set. The A/C control system does this automatically. The faster heat is taken out by the system, the more time the compressor is actually "on," and the greater the impact on the fuel mileage.

I'm not an A/c man, but I believe that GM cars in the past, and perhaps even today, had a more complex variable output compressor which used a "swash plate" to vary the output. With that design one could actually vary the output and not have to cycle it on and off to approximate the appropriate output. But I'm only 50% certain of that little factoid.

 

That all does not seem right to me. The a/c has to always be loading the engine when you turn it on, as far as i knew. i never turn it on at high rpms, i have seen that throw a belt in a car before (an older one though)

anyhoo not trying to argue just getting the facts sorted out

 

The car automatically turns the A/C of when you hit high rpms. Thats why when you floor it, then let off you notice instead of nice cool air, you get hot air. The car automatically does this to give you the extra power that the A/C robs.

 

"The a/c has to always be loading the engine when you turn it on, as far as i knew."

Sorry, but obviously you don't know far enough. I'll give ya' some education though. Some terms:

clutch-cycle pressure switch: A pressure activated switch that controls the air-conditioner-compressor clutch action to prevent evaporator icing.

clutch-cycle switch: An electrical switch, pressure or temperature actuated, that cuts off the air conditioning compressor at a predetermined evaporator temperature.

WOT Switch: An electrical switch that cuts off the air conditioning when the engine is at Wide Open Throttle (WOT).

These all affect whether the compressor is engaged or not, resulting in:

clutch-cycle time (total): The time between when an air-conditioner clutch engages and when it disengages, then reengages; a time equal to one on and one off cycle.

So the Compressor will shut off when the evaporator reaches a pre-determined low temperature. And this is affected by how much warm air is flowing over it and into the car, i.e., faster air flow, more heat transfer, the longer the compressor is "on," and the shorter the off period between "on" times.

Here's an activity for ya' After stopping and with the A/C running on a low fan speed, let the car idle, open the hood, and watch the A/C pulley/magnetic clutch for a few minutes. You will see it cycling on and off.

 

"The a/c has to always be loading the engine when you turn it on, as far as i knew."

= car, i.e., faster air flow, more heat transfer, the longer the compressor is "on," and the shorter the off period between "on" times.

f.

actually the slower the airflow over the evaporator, the more heat it transfers. That is why if you want to see the absolute coldest your a/c will get, you put fanspeed on the lowest setting at the coldest temp.

 

How significant is the difference in a/c engaged time between low fan speed and high? what difference would it be in mpg highway?

 

"what difference would it be in mpg highway?"

Depends on the ambient temperature and other things, as no one can even give you a precise answer as to the MPG impact of having it run on MAX COOL. But, as I said, it all depends on the amount of heat you are having the system extract. I would estimate that at the lowest setting the MPG impact is probably 20% of what it is at the highest setting, e.g., if the MPG impact is 3 MPG at Max Cooling, it would probably be around half an MPG at lowest cooling.

But if you were in Florida with 95 degree humid air, the difference would be less, as even at the lowest speed you are already putting a lot of heat into the evaporator, e.g., once you have "maxed out" the system, additional fan speed or temperature mix setting will not impact it a lot, as you are allready at the system's capacity (of course, this also means the turing the fan speed high will not make the car any cooler either, except for the cooling effect/sensation of more moving air over the skin). When it gets really hot and the A/C is maxed-out and cannot "keep up," I find sometimes a speed slower than the highest speed keeps me more comfortable. Why? Well, if it is so hot and humid that the A/C system is maxed-out at fan speed two, then the air coming out of the vents will actually be warmer at higher fan speeds. I find having a directed flow of cooler air go right onto me at these times preferable to having a faster, but warmer, stream of air flowing.

 

that show MYTH BUSTERS on discovery channel tested this....and their results showed almost no difference with either a/c on or windows down...they ran the same car with a full tank of gas around a track at the same speed ...once with the a/c on, and the 2nd time with the windows down and no a/c and they ran out of gas after the same amount of laps.......i do both myself...depends on the damn new orleans humidity....

Wrong, they got an additional 30 laps with the windows down and the AC off. Their first run was done with an OBD-II scan and estimated fuel economy based on the MAF readings and it indicated the same mileage on both vehicles. Also bear in mind that this was on an Explorer at sub-highway speed, so that could easily equate to 1.5 to 2 gallons of gas saved with the AC off...and at Focus MPG rates that could easily turn into an additional 50-60 miles per tank with the windows down and the AC off.

 

I like driving with my windows down.

 

I have no AC so the decision is easy for me. Window always down except for on freeway, and the fan helps a little when it is really hot. During summer when the average is in the 80s I have to suffer a bit, expecially while getting in the car when the temp can climb about 100 degrees