Originally Posted by
CONSO
The ' hump' for the upper deck helped to achieve a slightly higher mach number due to the famous " area rule " which simply states making the station to staion projected area " curve" as smooth as possible, so the hump slowly provided an increase in the transverse cross section area from the nose to the hump to the wing area.
When they incorporated the stretched upper deck for the 747-300 (and later the -400), the longer upper deck improved the area ruling enough that it compensated for the increased weight of the upper deck. In fact KLM had the longer upper deck retrofit to their 747-200s - and was pleasantly surprised to find it didn't affect the fuel burn at all. They basically got some extra seat miles for no increase in operating costs.