Originally Posted by ben@tirerack
Very good point regarding the point of diminishing returns. I read a test (way back in the early 90's I think) which compared results plus-sizing from a (then common) 15" wheel to +1, +2, and +3 sizing. The +1 was a little quicker around the track, the +2 was quicker yet, but started to feel a little harsher ride, and the +3 was not any faster than the +2, but harder riding.
This was one test of one car by one magazine, but it illustrates the point. In general, a particular vehicle stops gaining performance at a particular wheel size (aside from vehicles with extreme mods that may have special parameters).
Generally, newer cars are engineered around bigger wheels. So tire sizes that used to be silly are now common. 18" wheels on an E36 were 'too big' for pure performance, but are now the smallest thing offered on the E9x.
A little bit of flex makes a tire more progessive, too much sidewall flex makes it sloppy in transition. So there is always an optimal mid point between too big and too small. Honestly I'm not yet sure if it's 18 or 19.
You can get them with 18" or 19" from the factory and an inch must make some sort of difference. Between size and weight I can't see them as pretty close if the weight savings were 5-7 lbs. per corner. If one went to a lightweight forged 18" wheel and saved 5-7 lbs. per corner I would be surprised to see no performance gains..