depends how u hooked it up. are you using the new pcm to control it or did you leave the speedo hooked up the way it was? im guessing your using the new pcm. in that case its probably a matter of it being in the tune or something to do with your vss signal.. im leaning towards vss wiring because if it was a tune issue, either its not calibrated at all (unlikely) or there is a wiring problem...

this is just guessing since i dont know exactly what you did. post a little more info