The big boys refer to polar-moment-of inertia,which has to do with the mass of the wheel/tire combination,and where the mass is with respect to the center of rotation.They place the unit in a three-wire rotating pendulum,twist the thing up,and then time the frequency of "swing",to quantify inertial effects. Acting like four flywheels,its true that during "transient" episodes,when the car is accelerating or de-accelerating,either the engine or brakes must overcome the effects of the momentum due to the inertia of the wheel/tire combo.And I agree with whoever says,that,once up to a constant cruise speed,the mass of the units has little consequence to mpg,all else being equal.Composite wheels demonstrate the lowest mass,even against magnesium,have already received D.O.T.approval in the U.S.,and appeared on the market in the 1980s on a Chrysler
odge Shelby something or other.I tried to locate some for my exhibit and struck out.They seem to have bombed in the market,I don't know why.There are alot of things I don't know about!