Fuel economy comes from reducing BSFC (brake specific fuel consumption) which is not necessarily at lowest RPMs and or higher torque. There’s such a thing as overdoing it.
Ideally you’d size engine so it operates most of the time in the bulls eye area of BSFC map. The problem is that if you do that (which often leads to smaller engines), the transmission will need to downshift when extra load is placed on engine; as when accelerating or going up a hill. An occasional downshift doesn’t bother me, but is does most drivers.
I think a major reason for new 10-speed auto transmissions is to make shift increments smaller so they are not as noticeable and thus annoying to drivers.
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