I was having issues with 3 of my gauges on my '68 XR7. Oil pressure and water temp were reading too high. Fuel gauge was sometimes correct and sometimes worked backward Empty to Full. I tried new senders and verified groundings, but no change. Put this little regulator on and they are perfect. Not a calibration issue. Just a bad original voltage regulator. This dude goes on the backside of the gauge cluster....Not under the hood.
This is the modern solid state version, the one you want. Should last a long time.