[DonkeyBonker]

So it is a bit faster than the SR-71 classified top speed.

[DoughtyOne]

From 1959 through 1968 a decade-long research program employed the North American Aviation X-15 to explore hypersonic flight and technologies needed for space flight. The X-15's pioneering flights set records that still have not been broken four decades later, including a top speed of Mach 6.7 (4,520 mph) and a peak altitude of 354,200 feet (67 miles). While doing this it gathered engineering and scientific data needed to develop new families of aircraft and spacecraft, including the Space Shuttle.

http://www.sierrafoot.org/x-15/x-15.html