Abstract: In a locking device for use with a vehicle having a trunk compartment and a fuel tank filling outlet positioned adjacent and below a vertical lower wall of the trunk compartment, with the vehicle also having a closeable lid member which closes by engaging said trunk compartment lower wall, said device including a removeable cap closure member for the fuel tank inlet which tightens onto the fuel tank inlet by rotational movement and which has a slot in the exterior thereof, and a locking bar element having configured upper and lower portions, with the upper portion being adapted to fit over the top of the trunk compartment lower wall and engage the underside of the trunk lid, and with the lower portion having a lower terminus which is adapted to seat into the slot in said fuel tank filler outlet cap to lock thereby the cap in place and prevent its removal.