the fastest speed ever reached by a jet fighter is Mach 6.7, which is approximately 4,520 miles per hour (7,273 kilometers per hour). This record was set by the NASA X-43, an experimental unmanned aircraft, during a test flight in November 2004. The X-43 achieved this incredible speed by using a scramjet engine, which operates at hypersonic speeds by compressing incoming air before combustion. It's worth noting that technological advancements continue to push the boundaries of speed, so there might have been further developments since then. However, for the most up-to-date information, it's recommended to consult recent sources or news regarding aviation and military advancements.