I'm not sure exactly what you are doing but...
The tachometer output on fans is usually an open collector or open drain, but might have an internal pull-up to the fan supply voltage. The transistor pulls the tach output to ground to give 1, 2 or 4 pulses per revolution (depends on the fan). The output is not a variable voltage but is a train of pulses that your meter is reading an 'average' of in some non-useful way.
It sounds like your fan has the internal pull-up if you are reading a voltage between the tach wire (with it not connected to anything else) and ground.
What do you want to do with the tach output? You don't have to do anything with it (just leave it unconnected). If you are thinking of trying to read the speed by connecting the tach to the Pi, then, if it does have the internal pull-up, you'll need to convert the level to 3.3V. You could do that with a potential divider (two resistors) to drop the fan voltage down, or with a diode and pull up. The diode cathode connects to tach, anode to a 1k resistor to 3.3V and the anode/resistor junction to the GPIO.
If it didn't have the pull up (so is a pure open collector output like the one I was using earlier today) then it's easy to connect to the Pi with just an external pull-up or even the Pi's internal one on the GPIO pin.
ETA: Some fans' tachometers already have a potential divider inside, but will be set probably for 5V or 3.3V assuming they are 12V fans. To know which, you'll need to find the datasheet for the fan, or measure the tach signal with an oscilloscope (not a voltmeter).