when changing a wheel, too much talk about the weight.
not so much fuss though (if any) when we replace the tires.
maybe because grip and/or price is more important to some people or maybe because this factor don't come in mind.
but in the fight of reducing the unsprung weight, both wheel and tire weight count, so one could easily gain -5lbs from each corner of the car just by selecting a light tire.
-20lbs is a significant diet especially on unsprung weight
here are some weight examples i found for 245/45/18
29 lbs Falken FK452
27 lbs Re 050 Bridgestone
25lbs Michelon Pilot Sport 2
25 lbs Pirelli PZero
26 lbs Pirelli PZero Rosso
28 lbs Continental sport contact 2
24 lbs Continental sport contact 3
28 lbs GY F1 assymetric
29 lbs Dunlop Direzza Sport Z1
26 lbs Yokomaha ADVAN Sport
for info on more tire weight (stock size) u can check this
http://www.tirerack.com/tires/TireSearchResults.jsp?width=245/&ratio=45&diameter=18&sortCode=54050#445YR8RE050XL.