Should I just order a new Tach?
Something just doesn't add up. You mentioned that a mechanic sated your tach was accurate, so if they can be trusted then there is no reason.
As for checking your phone with a car, no expert here; but it's my understanding a phone does not use satellites to obtain position and speed, it uses tow locations. When your on a highway/road there could be several towers close by which should increase accuracy, how much, I don't know. On the water it would be my assumption that there would be less towers
