Originally Posted by
Alty7x7
The -400 had a combination of wing extensions plus the winglets, versus the -300. It was a 0.85 airplane, and could get up to 0.92 cruise above 40k.
The extensions and winglets didn't do much for cruise speed - the higher aspect wing helped overall drag but the winglets were close to worthless. Although the winglets had looked good in the wind tunnel, the real world performance was so disappointing that there was a serious proposal from the aero types to simply get rid of them. However Boeing liked the 'look' and how the winglets distinguished the -400 from the earlier 747s (and while the winglets didn't help much, they didn't hurt). So they decided to keep the winglets.
There was a meaningful improvement in cruise drag due to the stretched upper deck and it's area ruling improvement. When they retrofit a few -200s with stretched upper decks they actually got enough fuel burn improvement to more than offset the additional weight. Apparently the improvement in cruise drag with the stretched upper deck was a bit of pleasant surprise - they hadn't looked at it that closely during the development of the -300, but it made it a no-brainer to include it on the -400...