747-600X - actually, stretching the upper deck too much will make the aircraft less efficient. Why? Well, when the upper deck was stretched to its current length from what was seen originally, it was found to improve efficiency, because it increased the 'area rule effect'. I am not very knowledgable on this, however I believe it basically means that the fuselage around the wings should be narrower than the rest to achieve better efficiency. So, by having the upper deck extend until the wings, you make use of this area ruling. However, if you exten the upper deck further, behind the wing, then you give up this effect, and lose some efficency.
Perhaps someone with more knowledge of this subject could explain in more technical detail, but I hope this is helpful in someway
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