Maximum efficiency is when recirculation is on - no matter fan settings, no matter temperature selection.
Almost no air escapes through exit vents. But everything fogs up eventually
The slower the fan is when recirculation is off, the less "losses" there are. Loss is heat lost through exit vents.
Choose the temperature you are comfortable but keep fan slow. If you don't need heat and need maximum
range, set air direction to defrost only and set fan speed to minimum. reduce temperature setting as low as it goes
without fogging up the windshield. If you need heat, set it to floor as feet freeze first
So remember, start with cranking up the temperature, if that is not enough, crank fan speed up by one notch.
If it gets near perfect, reduce fan speed 1 notch. If that is bad again, bring it back up and reduce temperature.
Remember perfect fan speed for next trips.