Agree check actual speed with hand held tachometer. Does your tach reading remain steady while running or speed up and slow down erratically ? Does the tach speed change on it's own after running for a while ? (leaking adjustment cylinder)

Check the little rubber band that drives the tachometer. There is a hard to get to bearing behind the plastic cylinder tach pulley that can get stiff and affect tach readings. 

Here is the intermediate pulley spring Hud is referring to.

DO NOT I repeat DO NOT tear into this without talking to somebody who knows exactly what they are doing. This spring is under a lot of tension and can seriously hurt you if you are not familiar with how it comes apart and the tension.