I recall reading that reducing the weight of any rotating mass gets you the benefit of 3x that weight. I wonder if the actual number is 3.1416, pi.
Anyway, for what it's worth, the very fuel-thrifty Honda Civic HX has lightweight 14" rims that weigh about 11 lb. each. It's predecessor the VX had similar ones but I think they were 13". In other words, Honda thought it worthwhile to use lightweight rims on their fuel sipper, and racers now like to grab those rims.
Aside from the cost, I'm sure it's a boost for mpg.
Civic HX rim
