[aerohead]

Somewhere,I've got an SAE paper which deals with what you've asked about.Polar-moment-of-inertia is the physics part of your investigation.A special,3-wire pendulum is used to ascertain the value,and it has never been published info.As you've heard,the tire/wheel combo is essentially a flywheel,and the more mass,the more power required to accelerate,and decelerate.The SAE paper allows for an "equivalent-mass" to be estimated for the combos,as a function of vehicle mass percent.For an "intermediate-sized" car (used to be in the 3,500-pound inertia class ),a 10% weight reduction would net you a 1-mpg improvement.Thats alot of weight,and changing wheels and tires would never get you there.In town would yield the highest benefit,on the open road,it's basically a wash.Adding 1,200-lbs to my CRX only cost me 2-mpg on the highway.I'll try to dig out the article.