I'm the original owner of 1974.5 260Z.  The gas gauge started to read less than reality 15 years ago to the point a full tank read 1/3 full on the gauge last year.

Then it just sat on empty and never changed.   I did pull the sending unit and checked with a meter. All good, leading me to suspect the gauge.

I've recently started using a battery tender and while on a drive this morning noticed the gauge reading 1/3 full.  However, when I sat a light the reading went down to 1/8.

Green light, the gauge went back to 1/3 full.   I'm thinking the float is probably saturated but the larger question is what's up with the gauge?  Do I need to replace it or could it be in the wiring?

I've seen comments about connections, fusible wires, fuses, etc.   I know I haven't changed and fuses, nor has anyone been back behind the dash. In fact I really haven't experienced and electrical issues beyond this.

I thought I'd throw this out for discussion.   I'm wondering what next steps you guys would suggest.

It sounds like your voltage is getting too low.

Is it an internally or externally regulated alternator? If it is externally regulated (stock), test it per the BE section of the 260Z FSM. If car has been converted to use an internally regulated alternator, test per the BE section of the 1978 280Z FSM.