CONTINUOUSWAVE

[I am diagnosing] the malfunction of speedometer system. If I hook a pitot tube to the speedometer and draw a vacuum by sucking the tube, should I be able to move the needle on the speedometer? Thanks

No, you need pressure to check the gage, and the amount isn't that small. That is, you probably can't blow on the tube enough to make it register. Here's a typical calibration for Teleflex speedometer:

5 PSI = 20 MPH 
11 PSI = 30 MPH
19 PSI = 40 MPH 
30 PSI = 50 MPH

If you have a pressure gage or hook it up to a pressure source you can check your calibration.

Boat speedometers are just good old fashioned Bourdon-tube pressure gages, calibrated to read in speed rather than PSI.