[nighterfighter]

From the specs on their website: (http://www.vexrobotics.com/276-2156.html)

The optical shaft encoder can detect up to 1,700 pulses per second, which corresponds to 18.9 revolutions per second and 1,133 rpm (revolutions per minute). Faster revolutions will therefore not be interpreted exactly, potentially resulting in erroneous positional data being passed to the microcontroller.

So the maximum speed of the shaft being measured is 1133 RPM. Assuming you are spinning something faster, you will have to change the rotation speed of the shaft you are measuring.