I have been getting inconsistent reading for rate from the encoder get VI. The distance is good, but the rate will alternate between multiple values. The values are different depending on the Mode (1X, 2X, or 4X). I thought it was a difference between makes of encoders. It turns out that there is a LV problem that NI tech support has been able to reproduce. See my thread:
http://forums.usfirst.org/showthread.php?t=11004