Well the rule of thumb was about 1 hour for every 3000 nm flown, and the original concept was looking at ~10,000 nm range.
That was revised down to about 6500 nm. So with everything pristine and cruising at M=0.98, you could make hours plural.
But that's not what happens, as the reality of trying to do that with the ranges mentioned drives you down a bit in Mach number and also pushes you into fanjets that were at the time on the edge of feasibility.
You are operating right on top of the highest wave drag rise regime in flight. If you could use higher bypass ratio supersonic fanjets and supercruise it would be better - go about M= 1.2 -> 1.4
This is all rather easily determined and you can read a good paper on it here: Martin Hepperle's paper
But as he notes and others will, you will also get killed on time to climb and descend since you still have to operate in the same airspace as the slower traffic, so the time when you can find yourself getting that 20% extra speed margin tends to be attenuated. Maximizing it by lengthening the trip forces you to cruise slower to extend range and...you lose the speed advantage (not completely).
So in the end...maybe better off by an hour and some change?
Would passengers pay such a differential for such an advantage?
Nah. Boeing knew it at the time. I had friends working in that group. It was a concept airplane and they all knew it.
Some people think Mulally was just pulling a feint on Airbus to keep them off track while they went after the 787 design. I dunno, don't think Mulally was that smart. Maybe. It just sounded promising at the time, if...if...if.
Designing transonic engines and airplanes is a real challenge. You oughta try it sometime, I did it for a living many moons ago. Lot more difficult then the simple equations may make it look like.
Boeing people know that. Someday we'll get SupSonic airliners, probably not too long from now. But not transonic. There's no point, and it's right in the rough spot.
Yea, I was just noting that at 1 hr. saved for 3000nm, you would save at least 2 hours flying to Asia from the US, not whether the plane was or could be practical. But it is worth noting at the time that fuel prices were very low, so the actual cost of the fuel wasn’t a factor - but if high consumption affected range, then that would be a big deal.
As to saving a couple of hours being a big deal - I suspect that it would be for some business travelers, but not most tourist types. As it was, the move to the 787 was slick and well-timed, and could have really sunk Airbus if they didn’t need an extra 3 years (or so) to get its new systems working and if they had put in more than a token effort at understanding the batteries they were using.