Originally Posted by Parkanzky
You can't just consider the wheel weight, but also the weight of the tires. 20" tires will tend to weigh significantly more than a 19" tire and a lot of that weight is at the worst possible place.
Why would the tires weigh more?
For a given rolling diameter, increasing the wheel diameter actually reduces required tire sidewall. This reduction in tire sidewall material is most likely offset by the material needed to compensate for the structure stiffness. But, in my opinion, not to a significant extent.
Comparing different sizes weights on Tire Rack could confirm this...
For example Michelin PSS: 265/40R18=27lb 265/35R19=25lb 265/30R20=26lb 275/35R19=27lb 275/30R20=27lb