"Without belaboring a point, which is probably obvious to the reader, there are two features of the B-757/767 aircraft (and shared, of course, the B-747 and the B-777) which enable these airplanes to achieve the greatest benefit from the fuel-efficient engines and aerodynamic efficiencies These are (1) a computer which is able to perform complex cruise calculations and (2) an autoflight system, including autothrottle, which is able to translate the cruise calculations precisely.
The first-generation of turbojet transports such as the DC-8 and B-707 did not have autothrottles. If a pilot attempted to operate at maximum range cruise, it would be counter-productive due to the inherent instability at these speeds and the constant attention demanded of the pilot to thrust manage- ment. As a result, the "practical" minimum speed for these airplanes was Long Range Cruise (defined as 99% best economy). The fact is that flight manuals didn't even publish data for speeds less than LRC.
Second generation airplanes, such as the B-747, while incorporating an autothrottle system, were still unable to operate at maximum range cruise due to inherent inefficiencies of the autothrottle system. The automatic system operated the fuel controller though the same linkage used by the throttle levers in the cockpit. The system was difficult to maintain and thrust overshoots and undershoots were common. Much of the time at cruise was spent manually aligning the throttles. The practical minimum speed remained LRC.
The767-200 and the RB211 version of the 757 permit the fuel controller to change engine thrust through a pre-determined range without back driving the throttles. Although the system is still hydro-mechanical, more precise control of cruise is possible. Finally, with the advent of the true fly-by-wire system found on the B-747-400, 777, 767-300 and the Pratt & Whitney version of the 757, almost flawless control of the engines is possible. Speeds from Max Lift/Drag to the Vmo are fully usable."