I am going through the very same Problems with my 1993 F 250 4X4 . It is really Beginning to drive me out of my mind . when the ignition is on it reads Empty when its off its reads Half a Tank . I just Don't Know if i should replace the Sending Unit or get the Gauge Re-Calibrated as My Mechanic told me to do . He said it is the Gauge . But I Don't think its a problem with the Fuel Gauge .