Thanks.
One thing they don’t mention:
While maintaining the highest gear ratio and the lowest rpms is optimal for fuel efficiency, it is not optimal for the long term health of the engine parts - which are being starved of power by the low fuel consumption and therefore working harder and under more load and stress.
Maximizing your fuel efficiency by staying in the highest gear and lowest rpms is like a bicyclist refusing to downshift and accelerate his pedaling when encountering a hill. There will be a loss of power and increased leverage on the gears - maybe even snap the chain.