Finding RPM of DC motor
The RPM of the motor cannot be found using voltage and motor ratings alone.  You can estimate, but the load will directly affect the speed of the motor.

There is a way to measure the speed of the motor based on the current waveform.  Most DC motors have a 2-pole stator and a 3-pole rotor, so 6 current peaks can be observed for each rotation.



Run the current waveform into your ADC and write some slick software or use a comparator circuit to peak detect and you will have your speed.  You can see the measurement made and described on for(embed).