Did I pick you up wrong, did you say that you haven't taken delivery
yet ? In that case I would return the boat to the vendor to put right for you. there is a technical reason for the dissimilar Tacho readings, let them do a full repair, with a 6 month warranty before you take delivery
Don't worry, once you have the boat there will plenty of other things to tax your abilities. The good thing is, there's always someone here in the forum to help.