[jblackburn]

The compressor on the non-auto temp versions runs at full tilt all the time.

Yes, but it changes displacement according to fan speed and pressures. "Variable displacement". This is good though - you're not running the compressor at full displacement all the time and cutting it in and out with a compressor clutch that wears out down the road.

Also keep in mind that the coldest temp setting is auto recirculate. One notch up = fresh air. The recirculate button itself does nothing when the dial is set to max cold.

I definitely get lower MPG in town running my AC, but that's because the car has to work much harder. I usually turn it off to merge just so I don't have to floor it to cut it off. However, my comfort is a much bigger priority for me than MPG, and I love A/C.

On the highway, I notice little difference in MPG having the A/C on vs windows rolled down.

 

[jblackburn]

From a physics perspective you are generating the coolest air, and most dehumidifies air, by drawing the air a slow as possible over the evaporator. So again, the slowest fan speed tolerable is the way to go.

I've noticed that the vent temp doesn't seem to change between speeds except when I put it on max fan speed (a little less cold there). Makes sense - it tries to maintain a displacement where it's outputting a constant evaporator temperature. That, and I can hardly drive the car around town with it on max because the little hamster engine bogs so much.

I've never noticed mine switch between recirc and fresh air on its own - I can usually smell the difference in the air.

Funny realization - I have a long, steep hill out of my complex that I usually go down in 2nd gear. I've realized that I can control the amount of engine braking I get down that hill just by ramping up the A/C speed. If I put it on max, I actually have to use the gas going down the hill (normally I'd be riding the brakes to the stop sign at the bottom with A/C off entirely)

 

[jblackburn]

As an aside the MPG the car says it gets is always off vs my calculation. Anyone else see the same?

Yes, both generations.

Also, the A/C compressor in the Cruze is variable displacement. VERY little load is placed on the engine at low cabin fan speeds; load/drag goes up with fan speed.

I would not unplug it.