[Kevin M]

For freeway cruise, you only have to make the engine output match the combined resistance of drag, drivetrain loss, the portion of gravity that the grade of the road makes you work against (or with) and rolling resistance. So the energy you need to maintain a given speed is a constant (assuming the road grade is constant too) and must be equalled. Increasing tire diameter only affects this equation in two ways- it reduces engine speed which reduces the power it's producing at a given throttle input, and it reduces the torque transferring from the drive wheels to the pavement. These two things must be counteracted by the one thing that can- more throttle.

So theoretically I guess it could go either way, but instinct and experience tell me that a little more throttle is less efficient than a little more RPM. The best anecdotal evidence I can give is that my RS got 32-33 mpg on my courier run back in the day, but the wagon only gets 28 or 29 on the same route + level freeway driving from Sac to Vacaville. They weighed about the same, had the same tires essentially, but the car with the bigger, more powerful motor turning at slightly more RPM was 15% more efficient. The conclusion I drew was that the RS was cruising closer to peak torque and therefore was getting more energy out of the fuel and the wagon does.