3500kts is ~4028mph. Is Mach a set number or does it vary with altitude, temp, and other variables?
I would imagine they save full military power for outrunning SAMs and MiG-31s.
Mach does vary with air temperature, but the standard definition is...
At standard sea level conditions (corresponding to a temperature of 15 degrees Celsius), the speed of sound is 340.3 m/s[4] (1225 km/h, 761.2 mph, 661.5 knots, or 1116 ft/s) in the Earth’s atmosphere. The speed represented by Mach 1 is not a constant; for example, it is mostly dependent on temperature.
Since the speed of sound increases as the ambient temperature increases, the actual speed of an object traveling at Mach 1 will depend on the temperature of the fluid through which the object is passing. Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables. So, an aircraft traveling at Mach 1 at 20 °C (68 °F) at sea level will experience shock waves just like an aircraft traveling at Mach 1 at 11,000 m (36,000 ft) altitude at -50 °C (-58 °F), even though the second aircraft is only traveling 86% as fast as the first.[5]
https://en.wikipedia.org/wiki/Mach_number