Everyone knows MPG goes down when the AC goes on.

But, with the AC on, my friend swears I will get better mileage with the temp control turned fully CCW, the coldest setting.

He says that if you turn it to anything but the coldest position you are using both the AC and the heater, using more gas.

Is this true?

I was thinking if I used the AC but adjusted the temp to the highest temp I feel comfortable at I'd save gas compared to turning the knob to the coldest setting?

So, which is it?

This much is true, at least with any normal car with conventional controls. AC is either on or off with nothing in between. Heating uses waste heat from the engine coolent (antifreeze). A normal heat control is nothing more than a valve allowing more or less coolent into the inside of the car. Heating does not take more gas in the Summer when there is plenty of heat to go around. Heating will delay a cars warmup in the Winter and mileage will go down until the car is warmed up.

Anyway the bottom line is that I don't think your mileage would suffer by not having the control maxed out.

What car are your talking about ?

In my Prius, I set the cabin temperature to the highest I (or my wife, as the case may be) can tolerate with slight discomfort and not get drowsy driving or end up in a sweaty shirt .. Usually between 80 - 82 F. Then set auto AC, and let the car decide the rest.

Temps during the day are in the low 90's, in a low humidity climate. I'm seeing about a 5% mpg hit compared to temperatures in the 70's that will probably be my best results.

My next step is to tint the windows. I think with less direct sunlight coming through the big front window, the cabin comfort will improve considerably.

A car like the Prius has an electric variable a/c, so you just set the temp and it will adjust the performance of the a/c as needed. It can be full on, off, or somewhere in between.

A convetional car works as described above, where a/c is either on or off and temp is controlled by introducting warm air from the heater core (most cars operate like this). The car can cycle the a/c on and off as needed, but it's either on or off. So if you have the fan on the lowest setting it will cycle off more frequently to prevent the system from freezing (other conditions make it cycle as well, like radiatior temp getting too high) If you kick up the fan it may not need to cycle as much and you will use more gas.

So your friend is correct, if turning up the temp also causes you to raise the fan speed.

I've not been able to find any information on the in between aspect of the Prius AC. Do you have a linky? I've read that it has an eletric scroll compressor for providing AC when the engine is off and I've read about the electric coolent pump so the car will still provide heat when the ICE is off but I can't find anything about a variable compressor.

Only on very, very old conventional cars is the AC compressor engaged all the time. Modern 'conventional' cars has an electrically controlled clutch that engages and disengages the AC compressor as needed. Those with auto climate control engage the compressor less often when the seleceted temperature is higher. There may still be some mixing in of warmed air, but in part that is done to dry the air in combination with the cooling.

To answer your friend's misconception:
On 'conventional' modern cars, setting to max cold vs not makes no difference, since heating vs not heating has no effect whatsoever on fuel consumption. Heating is done by routing a portion of the coolant through a separate exchanger, and to the extent that is done the main radiator is 'taxed less'. In addition, on 'max cold' the compressor will run continuously, vs being disengaged periodically by the electrically operated clutch. Therefore, max cold will use more gas.
On a Prius, the AC compressor is not mechanically coupled ot the engine, it is driven electrically. It is driven as needed, and this means that on max cold, it is driven more than on a medium setting, and thus max cold will reduce your mileage. "Bleeding in" of heat may still occur in the Prius, as in conventional cars to dry the air. However, bleeding in heat may only affect mileage during the warmup period when the auxiliary electric heating element could be used. After the engine is wamr, all heating is done via engine coolant, to no effect on fuel efficiency.
Bottom line is, as far as I understand things, max cold will reduce your mileage more than an intermediate setting on AUTO climate control.

One of my future projects on my HCH is to rig a temporary indicator (light) connected to the AC clutch and routed into the cabin and have someone do a chart of on/off cycles (In a given amount of time) of full AC compared to...say the 4:00 position.

I'm almost certain that the AC cycles much less, but that would be the only way to be sure.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT

This reminds me. Anyone know how to rig a civics auto fan to stay on even when the car is auto-stopped.

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"

tbaleno:

On my Insight, if the fan speed is set manually, it will stay at that speed during Auto-stop. In econ mode or not.

Regards,

Not to hijack the thread but yeah the civic does that too. So why can't they keep it at the proper setting when set to auto

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"