Your statement ignores the point of the question and fact that the efficiency of an internal combustion engine is dependent on RPM. Try to go cross country in 3rd gear .
My attempt is to optimize MPG at a given commuter speed of 75MPH. This speed is probably higher than what the vehicle was optimized for. (Probably 60 or something).
Since a 5th gear to lower RPM is out of question, the only remaining option is the overall gear ratio. After more than 3000 miles I know now that 12% taller tires help about 2-3%. on my MPG.

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Good Patina Comes With Good Miles Driven.
Drive more polish less

220SEB 1963 (excellent driver with no rust but far from pretty)
300 TD Wagon 1981 (Daily Commuter Tractor)
911 MFI 1973

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