thats not enough to alter the signal. could be a few things off.

sensor could be going out, but its unlikely.
the sensor gear could be wearing off. all the ecu see is a pulse, not rotation or direction. could have a chipped tooth or something.

speedo could be going bad.

i say throw in another cluster from a friends car and recheck. what are you basing your speed off of to know what your real speed is?