Fuel Gauge not operable/truck runs out of gas after 25 gal.

What is your question?

bought used truck. recent onset. gas gauge reads empty and truck runs out of gas (literally) after 25 gal used. I had it in the shop (Firestone, they sent it to Chevrolet) for three weeks and they said they replaced the sending unit, but the problem persists. what do I do next? thanks