With an 18 YO fuel gauge, I would suspect the gauge! I had a truck with a gas gauge that was accurate between full and ? tank. Gauge never dropped below ? tank. 15 YO.
As Old Crow said, do not let it run dry! Fill it now, and you will know how much really remained.
With my truck, every gas stop was for a fill up. Reset the trip odometer. Buy gas at 200 miles, or run out around 250. (small tank)