THIS!!
There are bullions and bullions of posts on gauges on this site
View attachment 1715098940
If you are sure that the other gauges operate correctly......AND......reasonably accurately, it might be problems at the cluster connector (loose pins) a bad trace on the board, bad connection at the gauge studs, a bad gauge, problems in the wiring path / terminals to the sender, OR THE SENDER ITSELF which far and away is the most likely suspect
you can use resistors to check gauge accuracy, by substituting resistor for the sender
View attachment 1715098941
Hey Del, Look what I found!!! An original Miller Gauge tester!!!
Homemade gauge test tool, C-3826