Gas tank is full, however, the fuel gauge is on empty, why?

David this is typically caused by a failed in tank fuel level sending unit - however it is possibly a wiring or even a fuel gauge problem. The scan test will confirm which has failed so no money is lost on unnecessary replacement parts .. Test is pretty much bulletproof! .. Contact local auto repair facilities for scan test pricing - mention the reason for this test - you will not need a complete test of all modules - just fuel level / instrument cluster testing .. Repair shops scan tool should certainly be capable.