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Yes, the taller tires do reduce engine RPM, which reduces internal friction by reducing how many times the revolving parts have to turn over for each mile. Up to a point, this reduced RPM factor will increase MPGs, especially on the highway.
However, a point of diminishing or reversed returns can be reached by causing the engine to labor too much if / when the tires (effectively, the gearing) become too tall to handle. Only by trial-and-error can one find that "cry uncle" point, as each car is operated in different environments and circumstances: weight, wind, hills, etc.
Another thing: the clearances for the taller tires have to be watched. It is possible that even though a car can benefit from a taller tire based on the factors above, it may not have enough clearance in the wheel housing to take advantage of a taller tire. This also takes trial-and-error to find out. There is a point that the ever-larger tires start hitting the fenders from suspension flex, and a point that the front wheels can't make sharp turns because the tires are hitting things there.
I'm glad I got the larger "65" aspect tires. They seem to be justifying the switch for me.
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