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Hi Bruce,
Yep, that is right, to a first order approximation. The issue with steady state driving is that its not really steady state due to road imperfections, merging and road hazard avoidance. So weight will effect the mileage there too, but as you say the difference will be nearly insignificant.
On a Prius the effect is about half as much, as about half the rotational (wheels) and linear (total mass) inertias can be reused. In a standard car however, all that inertia is turned into heat by the brakes.
There are very expensive forged alumimun and or magnesium alloy wheels that can give you almost the same weight with the bigger heavier tires as with the smaller tires on cast alumimum alloy wheels, let alone steel.
Shop around, there may be some lighter weight tires, too. Although, they tend to have the lower tread life.
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