The way you draw it, the motor is always on, so I added a switch.
The problem here is that you put the rather large resistors in serial with the engine. Therefore, you will limit the current through the motor to 1mA max. The motor will probably not run with that small current.
Rather put the resistor bridge in parallel with the motor. Like that, the motor will not be limited in current and you will get the right output value.
I also added a 100nF condensator. Motors are known to produce voltage spikes and these will be flattened-out by the condensator. This wil save your Pi.
EDIT: As per comment from @Milliways, it is suggested to add a diode that should eliminate the backEMF that you get when you turn off the motor. I've added it to the diagram.
EDIT 2: Use only for very small motors!
The optimum would be to use an opto-coupler and completely separate the 12V circuit from your Pi. You will want to look into that.