Abstract: A cap for a vehicle gas tank, the cap having an opening formed therein closed by a door having a smooth, continuous convex upper surface which is hingedly secured to an annular support body. The door is spring-urged into sealing engagement with a gasket and is adapted to permit passage of a gasoline nozzle through the opening to move the door out of the path of the nozzle. The sidewall of the nozzle engages the convex upper surface of the door such that the periphery of the door is maintained in spaced relation from the wall of the nozzle to permit withdrawal of the nozzle from the opening. The door is spring-urged into sealing engagement with the gasket with sufficient force to maintain a seal when a partial vacuum is formed in the tank.