Vehicle AC question

I have been a bit more conscious about increasing my MPG in the vehicles lately and I try to avoid using AC but the afternoons are HOT. Does anyone know if there is any real difference in different settings of the AC being used? For example,when using the AC, the fan setting is at low but temp is set at 60 degrees. Because of the lower temp selected, does it still have the same effect if the fan was set on high at 65 degrees? I guess what I am saying does it even matter from a degree standpoint? Obviously fan on high uses more energy but what about the varying temp levels you can select? Another thing is this, my wife thinks it makes no difference to leave the rear AC on in the SUV if nobody is back there but I say it does.same thing for leaving dual AC on too. I try to tell her to leave everything off except just the driver like I usually do. I’d like to put this conversation to rest as to who is right or wrong. Yeah, yeah I know to never do a right or wrong conversation with the wife but I got to put my foot down here because of her new 1 hr round trip commute which I feel she is wasting gas on with these bad habits. So help me GRM forum members, you are my only hope. 

I think it depends to some degree on what sort of vehicle you're dealing with, but basically anything with a temperature set point - as opposed to something with a temperature dial ranging from warm to cool - is going to work to get to the set point, then cycle the system to keep it there. If you manually set fan speed, you may get there faster or slower depending on the setting; if you have automatic climate control, the fan will run hard to blow as much cool air in as it can, then slow down once the set point is reached. Obviously, every system has a maximum cooling capacity, but whether you use all of it or not is up to you; personally, I would think you'd want to use all of it to cool to the set point, then back it off once you're there, for maximum efficiency and comfort.

I can't say if there's an efficiency advantage to only cooling one side of the front cabin; if there is, it's probably minor. I would suspect that a separate system cooling the rear cabin, especially in a large SUV, would create additional thermal load that might reduce overall efficiency.

I also suspect the overall efficiency gained on a modern car will be pretty low.

EDIT: Another consideration is whether you're using fresh or recirculated air. Once cooled, recirc will certainly be more efficient in an automatic system, as the already-cooled air over the evaporator will require less work from the compressor and lower fans speeds.

Run your AC at a higher temperature , like 75.   Fans will run less.

Systems vary in how they're controlled. Probably the most peevish thing i've seen tons of people do is avoid the 'max' setting thinking it's going to cost them something. Generally that's just a E36 M3ty way of naming the recirculate function and you should ALWAYS be recirculating (in terms of efficiency) unless your interior air is hotter than outside air (such as first couple of minutes after vehicle sat in sun for a long time). 

The blower motor is a relatively small load compared to the compressor. However, running the fan on a low setting will generally let the evaporator core temperature (and thus pressure) drop to a point where the low pressure cutoff switch will disengage the compressor to keep ice from accumulating on the evaporator core (thus blocking airflow), and that in turn will often disengage the electric fan for the condenser core because a lot of them are triggered by high side pressure coming out of the compressor. So, that's where the majority of the savings from running the blower on a low setting will probably come from. 

Personally, I'd rather have the AC on full blast in an efficient car than try to put lipstick on an MPG pig  by turning my AC down. This is coming from a South Texan so your climate may vary. I put a 24000 btu AC in my 500sq ft insulated shop just so i could keep temp inside in the mid-80s. I'm sure as hell not going to turn my car AC down over the matter of a few bucks a week. I'd rather be in dry clothes than have a few more dollars. Again, this is Texas talking. 

Every car I've had (as far as I know) runs the compressor non stop when the A/C is on, regardless of fan settings, unless it freezes up.  The only reason to mess with the fan settings is comfort.

FWIW, Driving habits are going to make an impact an order of magnitude larger than A/C usage habits. 

Sine_Qua_Non said:

Another thing is this, my wife thinks it makes no difference to leave the rear AC on in the SUV if nobody is back there but I say it does.same thing for leaving dual AC on too. I try to tell her to leave everything off except just the driver like I usually do. I’d like to put this conversation to rest as to who is right or wrong. Yeah, yeah I know to never do a right or wrong conversation with the wife but I got to put my foot down here because of her new 1 hr round trip commute which I feel she is wasting gas on with these bad habits. 

The problem here isn't her habits.  Its that she has a 1 hour round trip commute in an SUV that is presumably fairly large as it has dual AC (we are talking separate condenser/vents right, not just front vents piped through the center console?)  Seems like you would be better off getting her into an efficient vehicle where she can keep the climate control set to her liking.

I think janerampl is a canoe.

I concur with P.O.D.

Even if you were right it's not worth a couple of bucks to argue with the wife.  As ProDarwin said, get her a car that's more suitable for a commute.  Unless she has to off-road or tow a boat to work.

Recirc is more efficient than fresh air once the inside is cool, because you're working with already cooled air.

Low fan speeds give you more cooling than high fan speeds, because you are reducing the amount of air going through the evaporator, so more BTUs are removed per unit of air.  (You might cool down to 38F on low fan speed but only cool down to 60F on high)

Pete. (l33t FS) said:

Low fan speeds give you more cooling than high fan speeds, because you are reducing the amount of air going through the evaporator, so more BTUs are removed per unit of air.  (You might cool down to 38F on low fan speed but only cool down to 60F on high)

I don't think that's how this works - you want as much flow as possible so as to impart the maximum amount of heat in to the refrigerant, sending it back to the compressor as hot as possible.  The high fan speed will give you the most *cooling* (ie, total amount of heat removed from the cabin), but the low fan speed might give you more cooling *per watt of fan power consumption*.  More airflow is always better if you want heat transfer.

Matthew Kennedy said:

Pete. (l33t FS) said:

Low fan speeds give you more cooling than high fan speeds, because you are reducing the amount of air going through the evaporator, so more BTUs are removed per unit of air.  (You might cool down to 38F on low fan speed but only cool down to 60F on high)

I don't think that's how this works - you want as much flow as possible so as to impart the maximum amount of heat in to the refrigerant, sending it back to the compressor as hot as possible.  The high fan speed will give you the most *cooling* (ie, total amount of heat removed from the cabin), but the low fan speed might give you more cooling *per watt of fan power consumption*.  More airflow is always better if you want heat transfer.

The question is not about "most BTUs removed" but "least engine power drawn", and running relatively low high side pressure will get you that.

Besides, your theory that higher fan speed = more BTUs removed does not factor in that we are trying to make humans comfortable, and a small amount of 40F air feels better to a human than a large amount of lukewarm air.  That's the "ice cold air" people are looking for.  And as a bonus, you can actually get to hear yourself think instead of getting nothing but fan hiss