A protective cover for a filler neck is designed to discriminate between different sizes of fuel nozzles. The cover includes a body portion and a movable door that covers and exposes an opening in the filler neck. The body portion has a pair of spaced body protrusions that form a gate, and the door has a ledge that may also be formed by a pair of spaced door protrusions. The protrusions on the door can be easily moved to an open position by pressing a tip of a fuel nozzle against the protrusions. The gate and the ledge are configured so that they can accommodate the fuel nozzle for only one type of fuel.