Circumstances, and time and distance data from the Coast-to-Coast record flight on March 6, 1990 seem to indicates that the top end of the SR-71 was in the neighborhood of Mach 3.3.
When considering that the record run preceeded the first deactivation of the SR-71 program it's safe to assume that had the acft been capable of producing a higher mach number it would have been done, if for no other reason than to have the bragging rights.
Its doubtful that Mach 5 or 6 would have been possible.
Looks like the X-15 was the first one to ever hit Mach 6 or better
ipmsstockholm.org/magazine/2004/03/stuff_eng_x15.htmSample paragraph:
The X-15 become one of the world's most important research airplanes. It was the first airplane to reach Mach 3, Mach 4, Mach 5 and Mach 6. On October 3, 1967 Pete Knight flew the X-15 and established the world absolute speed record for winged vehicles of Mach 6.70 or 4,520 miles per hour (7,232 km/h). This is twice as fast as a rifle bullet. More than 35 years later this speed record still stands for an airplane. Only the space shuttle has flown faster. The X-15 first flew in 1959 and made 199 flights during the next nine years. The X-15 was constructed to fly higher and faster than any other airplane. The X-15 exceeded 50 miles (80 kms.) on 12 occasions. On August 22, 1963 Joe Walker flew the X-15 to 354,200 feet (67 miles or 107 kms.) to set the world altitude record for winged vehicles. Research from the X-15 project was vital to the design and construction of the space shuttle.
Contrails of that "ram-jet" powered vehicle over L.A. in recent years (supposedly flying out of Area 51) may indicate that something newer may be capable of mach 5-6?
Then there's this from Defense News Editor, Vago Muradian
11 June 07 An SR-72?
Lockheed Martin's New Mach-6 Spy PlaneBy Vago Muradian
Ten years after the U.S. Air Force retired the SR-71
spy plane, Lockheed Martin's legendary Skunk Works
appears back at work developing a new Mach-6
reconnaissance plane, sources said.
The Air Force has awarded Lockheed's Advanced
Development Projects arm a top-secret contract to
develop a stealthy 4,000-mph plane capable of flying to
altitudes of about 100,000 feet, with transcontinental
range. The plan is to debut the craft around 2020.
The new jet - being referred to by some as the SR-72 -
is likely to be unmanned and, while intended for
reconnaissance, it could eventually trade its sensors
for weapons.
The Air Force is working on several programs to
improve its global intelligence-gathering. Satellites
offer global coverage, but the ones with the highest
resolution operate on largely predictable orbits, and
many countries have mastered the art of hiding from
them. Moreover, China's successful anti-satellite
missile test in January hinted that U.S. satellites
might become vulnerable.
The new aircraft would offer a combination of speed,
altitude and stealth that could make it virtually
impervious to ground-based missiles, sources said. Even
the SR-71 is said to have evaded hundreds of missiles
fired at it during its long career, although some
aircraft sustained minor damage.
But experts say enormous challenges remain. First, the
SR-71's top speed was about 2,200 mph. Pushing a plane
at twice the speed in the thin air of the upper
stratosphere would require exceptionally powerful
engines. Second, friction at high speeds could reduce
stealth.
"An aircraft with these characteristics could prove a
potent response to anti-satellite weapons," said Loren
Thompson of the Lexington Institute. "If U.S.
reconnaissance satellites were lost, an SR-72 could get
to areas of interest quickly and provide persistent
surveillance in place of the satellite."
And don't bother asking the Air Force or Skunk Works
execs about their work. Neither is commenting and Skunk
Works is skipping next week's Paris Air Show. "As a
matter of policy, we don't talk about classified
programs - whether or not they exist," said Lockheed's
Tom Jurkowsky.
Pat Perry