The maximum and cruise speeds listed for North American XB-70 Valkyrie are:
- Maximum speed: Mach 3.1 (2,056 mph, 3,309 km/h)
- Cruise speed: Mach 3.0 (2,000 mph, 3,200 km/h)
Why is there such a small difference?
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From the excellent point in the comments, the XB-70 was designed for one thing; speed. It was, until the Blackbird, the fastest manned air-breathing aircraft. It was intended to outrun Soviet interceptors like the MiG-25 and fly higher than ground-based anti-aircraft weapons of the day, making it a possible first-strike weapon. As such, it wasn't going to loiter around at 80% throttle.
A second point is that since the B-70 project was cancelled for several reasons (cost projections, development of ICBMs and of high-altitude fast-tracking SAMs), the two prototypes were used primarily as testbeds to push the envelope of air-breathing craft. As such, data regarding their fuel efficiency curve was secondary to more basic problems of aircraft design for hypersonic aerodynamic flight.