You correctly point out that the power output must be equal in this scenario. I was suggesting that this oversimplifies the equation, as it incorrectly presumes the specific fuel economy of the engine is constant across its RPM range (i.e. ignoring the characteristics of its powerband). That being said, even if the engine has a slightly higher efficiency ratio at a higher RPM, as it comes closer to peak torque output, this does not mean the engine consumes less fuel overall when spinning at a higher RPM. The presumption that a taller gear will achieve better economy for a given vehicle is generally valid as long as the engine remains within its powerband (e.g. not "lugging"). Is that fair to say?