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I've just switched from 185/60 tires on 14" rims to 155/80 tires on 13" rims -- same overall diamater. The new wheels weigh 2.5 lbs less, and the tires weigh 4 lbs less.
But I believe tire weight is much more important than wheel weight. It's all about where the weight is concentrated as radius is squared in the inertia formula. ie, my 13" wheels have the weight concentrated mostly at about a 6" radius, but the tires at about 11" radius. When you square the radius, for this size, it works out to the tire contributing 80% of the inertia. I estimate that I should need about 20 lb-ft^2 less torque to achieve the same acceleration.
I just mounted these 2 days ago, and the car feels like it accelerates from a stop much more easily. But any improvements in FE may also be due to other factors like less aero drag, but I think the bulk if any will be due to the lower inertia. I'll know more soon when I fill up next.
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