On a hot day and traveling in a car, which method of keeping the occupants comfortable with the A/C is more energy and fuel efficient?

  • Set the thermostat to a lower temperature and keep fan speed to the lowest setting

  • Set the thermostat to a slightly higher temperature and compensate by setting the fan to a higher speed so you can feel cool enough with the breeze

  • FuglyDuck
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    1410 months ago

    Likely doesn’t matter. Like. At all.

    Ac is likely chilled to a given temperature regardless of what the final temp is called for.

    The air is cooled by blowing it over a radiator on the cold side of the AC’s compressor, the radiator then absorbs heat from the air (putting it into the coolant.)

    As long as you’re not putting more heat into the coolant than the chiller can extract, it’s only going to remove so much heat- more if it’s slower, because it’s in more time, it then it’s spread across less air, if that makes sense.

    Regardless, the way temperature is controlled is by mixing it with additional ambient-temp air. (Either cabin air or outside air.)

    So technically, it would be most “effecient” to have full cold, but (as already said,) the ac system is effecient enough it basically doesn’t matter.

    Going the other way doesn’t matter at all because it uses waste heat from the engine in a bypass loop. That said, if you have a cold car you’ll slow down the engine warming up by turning on the heat. (You can see this when it’s very cold by warming up the engine and watching the temp gauge when you turn on the air.)

    EVs are a bit different- heating from cold start is done with electric heaters. Once the car is moving and the motors are hot, they typically use waste heat from there.

    I assume there’s little difference between ice and ev in cooling,