johnwasser:
The 'speed' output pulses twice for each slot in your slotted disk.
Read up on pulseIn() or the PinChangeInt library; you'll want to measure the time between the two pulses to determine the speed. It's not very clear how that would work programatically because, if the sensor is outputting high normally and blinks low at the beginning and end of an interrupted light beam, how do you tell if you're measuring the length of a blocked or unblocked beam?
Honeywell has the worst documentation for its stuff.