Originally Posted by elp_jc
There's always a point of diminishing returns on everything. On wheel size, seems like it's between 18s and 19s, depending on track, as previously said; no one tire/wheel combo is best for everything.
By the way, stock 19s are lighter than 18s because they're forged, but forged 18s would be lighter than forged 19s in same design, but a 5-spoke design would be even lighter. Then you have the tire width, aspect ratio, and wheel width to mess with
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.