A hub cap for a wheel hub having a cylindrical housing with a closed first end and an open second end adapted for attachment in an air-tight sealing arrangement to the outer end of a wheel hub to form a closed air system in the interior of the wheel hub. An air pressure gauge in an air flow connection with the closed air system is operable to provide a visual indication of the air pressure in the closed air system. The air pressure gauge includes a dial face and a pressure indicating needle moving relative to the dial face in direct relation to the air pressure within the closed air system. The dial face includes indicia thereon to assist an observer to readily identify a breached seal condition within the wheel hub.