On my 1988 ford ranger 2.9l fuel tank full but gauge reads empty, but when fuel tank is empty the gauge reads full

SOURCE: fuel gauge stuck on full

need to replace the sending unit

SOURCE: Fuel Gauge not working

The switch that you use to change tanks is probably faulty - the switch supplies power for the sending unit in the tank, and to the transfer switch for the gas lines - use a meter to check it

SOURCE: fuel gauge not working

yea fords with two tanks are known for getting stuck if you dont switch the tank constantly. you have to get to the valve that switches the two tanks, and free it up. it definitley isnt anything but the valve.

SOURCE: gas gauge reading empty even when tank is full

Usually if the gas gage does not work yes it is true it may be the sensor if it is electronicly controlled. the sncor will more than likley be in the tank. here is how to remove your gas tank.

1. Before servicing the vehicle, refer to the precautions section.
   
2. Relieve fuel system pressure.
   
3. Drain the fuel tank.
   
4. Remove or disconnect the following:
   
   • Negative battery cable
     
   • Fuel filler and vent hoses
     
   • Fuel tank skid plate
     
   • Fuel tank wiring connectors
     
   • Fuel supply and return lines
     
   • Fuel tank
     
   • Fuel pump assembly
     Fuel pump assembly mounting
     

1. Install or connect the following:
   
   • Fuel pump assembly
     
   • Fuel tank. Tighten the bolts to 27 ft. lbs. (36 Nm).
     
   • Fuel supply and return lines
     
   • Fuel tank wiring connectors
     
   • Fuel tank skid plate
     
   • Fuel filler and vent hoses
     
   • Negative battery cable
     
2. Start the engine and check for leaks.
   

SOURCE: 2001 hyundai sonata 4 cl can't pump fuel into tank @ normal rate

The sending unit in your tank is going bad replace it.