Electrically speaking, the blower runs at the same speed on MAX as it does on HIGH. You might get a few more RPMs out of the motor on MAX with the windows closed because you're not building up pressure inside the vehicle. When you're pulling in outside air and the windows are closed, that air has only little holes to escape so pressure builds slightly inside and places more of a load on the blower, causing it to run slower.
On MAX, you're recycling the inside air so no pressure is being built up and the blow runs faster due to less of a load.
Of course when you're on MAX it sounds louder because you now hear the suction side of the blower. This gives the perception of the blower running faster too.
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air conditioning
