What wheels are you running? It is rare to find an 18 wheel/tire combo that is going to be lighter than the svt rims (19 for E1 and 20 for those prior).
I went from 17s to 16s last year (the 17s were 24lbs and the 16s were 15.5) and then did some 0-60 runs w/ my g-tech. I dropped 3-4 tenths from my times with the 17s. Also note that tires(of the same size) do not weigh the same and when you go to a larger tire, the tire is also going to weigh more.
