fuel gauge problems

Only way to tell for sure is to rig up a test circuit, one way to do that is to yank the cluster and unscrew the stud nuts on the gauges. Pull the VR for certain. This will isolate the gauge studs from everything, and you can hook clip leads to 'em.

If you then wire the two gauges in series, and hook to an power supply you can vary (AA cells, in series, etc) the two should read the same

That is, power supply + to temp gauge + ----- temp gauge sender terminal to fuel gauge + ---- fuel gauge sender terminal back to power supply neg.

So if the gauge you think is good reads 1/2, so should the second.

Here's some photos, 3V battery, first the gauges in series, then the two gauges in parallel. You can see that they read substantially the same, proving that either both are fairly accurate, or both are fairly off by the same amount