If the vehicle suffered an impact, it probably affected the meter. Here's the deal:
This meter is a pretty primitive piece of work, and it is calibrated by the location of a small magnet which is glued into its case. The glue is brittle. If the magnet gets dislodged, it becomes wildly inaccurate (this in addition to the inherent inaccuracy of the circuit that feeds it). The magnet doesn't actually fall out, because the case itself is steel. So what you need to do is to take out the cluster, take out the voltmeter, open up its back, and try to reglue the magnet in the proper locationo on the inside of the back. You'll have to experiment, but the calibration basically is done by moving the magnet forward and back in the case. You can use a sticky glue like shoe goo, or prehaps a tacky craft glue, to hold it in place when you get the right value.
You can connect wires to the naked voltmeter, and use your battery or some other source whose voltage is known accurately, and adjust it that way.
Once it's calibrated, it will still lie when you turn your heater fan on high, but it will do it with some consistency.