I have a 67, not a 69 but maybe the manual will give you the expected values, or you can dig it up online. You could also connect it to the car and swing it up and down and see what the gauge reads in case the float itself is not floating as it should or is hitting something (give it time to react). Wiggle it a little, too, it may have poor contact with the resistance windings. If all checks out with the sender then I guess that you need to look at the gauge. A nice new fresh one is tempting, but I don't always trust the quality of replacement parts. You are wise to check it out first before just replacing it.