The 747 efficiency drops off sharply above M0.85 cruise. I remember reading that the VC-25 (747-200 based) 100% thrust cruise speed is around M0.96 with conside

The 747 efficiency drops off sharply above M0.85 cruise. I remember reading that the VC-25 (747-200 based) 100% thrust cruise speed is around M0.96 with considerable range penalty. The 747-400 has newer engines but pretty close 100% cruise speed. This is above the approved Mmo for the 747 and not authorized operationally. Test pilots have reported that a Gulfstream G550 airframe can handle supersonic flight, and the BR710 turbofan engines can handle it. The limit is the engine intake design. With an optimized intake, its entirely possible the fuel consumption at M2.2 would be comparable to ~M0.98