Why people jump to negative conclusions based off of what others have tested and proved to be positive is sad. He's reducing weight upon the rolling resistance of the vehicle, by quite a bit. Not only that the contact patch between those two tires with the road is also reduced. If you guys are going to flame pick a better fire [

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If he's running similar pressure in the tires the contact patch will be about the same. A difference in tire compound and sidewall construction can have an effect on mileage and there are purpose built tires designed to improve mileage, albeit at the expense of handling.
Smaller wheels are likely to be lighter and have a lower moment of inertia which can improve acceleration and braking and may have a very small benefit in mileage, but the mileage effect would be impossible to notice in the tank-to-tank noise. At constant speed the weight and moment of inertia difference would be make almost zero mileage difference.
Again, claiming 10% improvement is out of the question, it just is...
Brian