It is not clear what type of fan you have...
Is it brushless?
If it has brushes you could add some resistors, a capacitor and a zener and a diode to make it a safe input...
My guess is that without load resistor, you will not get useful readings.
Without protection resistors and a zener, you may easily destroy your analog input. A diode might be a good idea to protect from reverse voltage (due to EMI from the motor or wind from behind).