Not sure if this will help but at least give you some perspective. I just received today a set of actual OEM 17" Equinox wheels, with TPMS sensor (for a dedicated set of winters I got off ebay).
I weighed myself on a digital scale with and without holding the Wheel.
The Factory 17" OEM Equinox Wheel, model year 2010, 2011, weighs 22.5lbs
I imagine an 18" can't be much more, maybe another 2-3 lbs or so???