If you read the encoder twice and note the time between then I would expect:
Should give you the speed in RPM. 720 should be the number of encoder ticks per revolution.
Using an NXT SERVO Motor what I get out of that appears to be about double the speed. Free running with a command value of 25 I count about 5 revs in 10 seconds (30 RPM) and a calculated speed of about 60 RPM.
Anyone have any ideas?