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First, the differences between tires (meaning make and model) is much larger than the differences due to tire size.
A larger tire has more load carrying capacity and a tire shop is unlikely to mount a smaller tire because of the legal issue.
A larger tire generally has better RR - all other things being equal - but, again, the differences between make and model are much larger.
The air resistance is a smaller tire is negligible.
So the best use of your time is to research tires of the specified size.
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