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?
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Set the thermostat to a lower temperature and keep fan speed to the lowest setting
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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
The one detail I haven’t seen in the other comments would be to recirculate the air inside the car, so the car doesn’t have to cool the hot air from outside, but instead only cool the already cooler air from the inside. To be the most optimal if your car has been in the sun you should make it draw air from the outside until it’s a little cooler than the outside then recirculate