re: OT mechanics. Replacing cluster guage

Does it have a digital odometer or an older analog style? If it's a digital than it won't matter because it gets that info from the ecm in the vehicle. The mileage suddenly dropping like that, and it needing a new cluster makes me think there are electrical issues. The only ways the mileage would drop like that would be due to someone going into the ecm and changing it, or mosre likely the ecm got replaced with a used one from another vehicle.