anyone know the benifits of lightweight wheels?

[fu_im_from_texas]

I've been thinking about this question for a few days. My gut reaction was: lighter wheels will make a significant improvement in accelleration and mpg. My calculations show the opposite. Maybe my logic is flawed? Here's what I've got so far.

Consider two cases:
1) Totally stock Yaris from 0-60mph
2) Yaris with lightweight rims from 0-60mph (NOTE: I'm assuming identical tires)

The easiest way to compare the two is from an energy standpoint.

Rotational Energy of a Wheel = .5*Mw*(Rw^2)*W^2

where:

Mw= Mass of the wheel
Rw= radius of the wheel
W= rotational speed (Radians/second)

For Case 1)
Mw= 15lbs. I base this on the wheel/tire combo weight of 33 lbs. tirerack.com says the tire weights 18lbs. 33-18=15
Rw = 7.5 inches. 15in diameter / 2
W = 92.15 (RAD/s). @ 60mph 1mile takes 60 sec. for stock tires tirerack.com says 880 rev/mile so 880/60 = 14.6Rev/second *2Pi= Rad/s

convert to metric

Mw= 6.8kg
Rw=.19m

Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*6.8*(.19^2)*92.15^2
=1042 watts = 1kWatt

For Case2)
Mw=10 lbs. = 4.5kg This seems like a reasonable lightweight 15 inch wheel
Rw=7.5 inches = .19m
W = 92.15 RAD/s

Rotational Energy = .5*Mw*(Rw^2)*W^2 = .5*4.5*(.19^2)*92.15^2
= 690 watts = .69 kWatt

1- (690/1042) = 33% less energy to accellerate the lighter wheel. This sounds like a lot untill you compare it with the total energy of the car @ 60mph.

Energy of Yaris = .5*M*(v^2)

Where:
M=mass of yaris=2200 lbs = 998kg
v=velocity=60 mph= 97 km/h

Energy of Yaris = .5*998*(97^2) = 4695 kWatts

so (Energy of Wheel) / (Energy of Yaris) @ 60mph is 1/4695 = .0002

.000002% of the total energy is insignificant.

Did I make a stupid error in my calc? i would have guessed lightweight wheels would make a big difference...but I proved the opposite. someone help? ...show me the error...