Originally Posted by G35fromPA

Keep in mind that raw weight is not the only consideration here. It's also where that weight (mass, really) is concentrated, otherwise known as the "moment of inertia." The 18's weigh a few pounds more, yes, but that extra mass is 1" closer to the center of the wheel than that of the 19's. So, in actuality, the moment of inertia of the two wheels may be pretty close, meaning the torque required to start and stop them may be about the same.

So, if you imagine the wheel to be a hollow cylinder for simplicity's sake, the Moment of Inertia of a hollow cylinder = 1/2 x Mass x (Outer Radius^2 + Inner Radius^2). As you can see that as the radius increases, the moment of inertia goes up as square of the radius. The mass increase is just a direct multiplicative factor.

If you assume (again for simplicity's sake) that all the mass is concentrated in a 1" ring around the edge of the wheel (i.e. from 17" to 18" for the 18" wheel and from 18" to 19" on the 19" wheel), and do the math, here's what you get for moment of inertia:

Conversions:
17" = 1.42ft
18" = 1.5ft
19" = 1.58ft


19" rear wheel: (1/2) x 20lb x (1.58ft^2 + 1.5ft^2) = 47.6 lb-ft^2

18" rear wheel: (1/2) x 24.5lb x (1.5ft^2 + 1.42ft^2) = 52.1 lb-ft^2

So, al in all, the wheels should perform pretty closely in terms of real world acceleration, deceleration and handling. Maybe a 1-2 lb reduction in weight for the 18's would bring it to the point of being a wash.

Originally Posted by Sukairain

G35fromPA, the formula is correct but the center of mass on the 19" rims is actually not that close to the edge as compared to aftermarket wheels with fat lips. The stock 19"s have minimum lips and the shape of the outer rim is like a 18" with 1/2" extension right at the very edge to make it firt 19" tires.