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A lot of MPG losses are friction losses.
These losses eat only a percentage off of the total MPGs, but since they are added every revolution, they do add up.
Most engines rev fine between 2 to 3k RPM, where the torque curve is stabilizing (going more horizontal).
Any additional rev, adds to friction losses.
If you need extra power, there is no other way, than to rev higher; as more revs creates more power, but it also creates more friction losses.
On most engines there is a peak somewhere between 4-8k RPM, where the torque curve tapers off.
That's where the engine's gain in RPM is offset by those same friction losses, and where every additional rotation does add HP, however, it subtracts more power through losses.
For that reason, for any economically minded person, it makes no sense to ever accelerate beyond the torque band (not the HP band).
And this is only when fast acceleration is absolutely NEEDED.
If it's not needed, the lowest RPM range for the acceleration, without lugging the engine is most desired.
Do be aware, that keeping revs REALLY low, like at 1500-2500rpm, can break the engine when trying to accelerate without downshifting to a lower gear; even if the engine can cruise just fine in that RPM range.
I blew a head gasket once, for pressing the pedal to the metal at 1500RPM.
(I thought I was in automatic, and was trying the automatic to downshift me, but it kept in 6th gear; which lead to the gasket leaking coolant fluid in the car).
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