Defective fuel sending unit.
In the gas tank
you should go to a car shop for that one!:)
You might plan to refuel when your fuel gauge reads one-eighth of a tank of gasoline. Some people choose to refuel when the gauge reads one-quarter of a tank remaining.
You probably have a faulty fuel gauge sending unit.
The fuel gauge on a 1985 Chevy Silverado will read full all the time if the sending unit is defective or the wire has come loose. When the tank is full, the sending unit normally shuts off which cause the gauge to read full.
Generally the fuel gauge sending unit located in the fuel tank is at fault for incorrect fuel level readings.
Simple check is to ground the fuel sending wire at the fuel tank.If the gauge reads FULL then it is a bad fuel sending unit and you need to drop the fuel tank to replace it.If the gauge reads empty when you ground the wiring then it means you have a bad fuel gauge.I would lean strongly towards the sending unit be bad my 87 the gauge started reading about 1/4 tank less before it failed.
Bad sending unit. This is what reads the tank level and sends it to the gauge.
sounds like you have a bad ground wire on frame by the fuel tank. small black wire coming from the fuel tank and screwed to the frame.
Sounds like the transfer pump in the rear tank is bad and you are running out of fuel in the front tank. The fuel gauge reads both tanks and gives you an average between the two. So, if the front tank is empty and the rear tank is full the gauge will read aprox a half tank.
you need to replace the sending unit in the tank!
The sending unit is either defective or it is hanging on something in th tank.