A relief vent assembly for a fuel storage tank includes a vent body defining a chamber to provide a substantially axially smooth and unencumbered path of flow for pressurized fluid to vent out from the tank. Such a path of flow allows the pressurized fluid to remain laminar throughout approximately the entire path through the chamber, maintaining a high flow rate. The vent assembly includes a crossbar that supports the rod of a lid that covers a distal margin of the vent body. The crossbar is disposed substantially at the distal margin of the vent body to prevent any premature turbulent flow of the pressurized fluid, which would slow the flow rate. A widening tapered bore of the chamber increases the flow area and serves to offset the area occupied by the crossbar.