Originally Posted by 4sevens.com
Of course a certain amount of sidewall flex is needed - but the question is HOW MUCH? There is an optimum slip angle for max grip which depends on ALOT of things. The type of tires you're running (slicks, r-comps, streets have VERY different optimum slip angles). Also your alignment, weight, suspension, geometry will all matter. Just because you threw out a race car example doesn't make much of a point.
I have first-hand clinical results - running 18's versus 19's back to the back on the same car, same day with same alignment and same rubber I was able to shave off almost 4 seconds at road atlanta - thats what I have to report from experience from MY car - I don't speak for all.
If you attribute the difference between 18s and 19s for a 4 second gain per lap, you've lost all credibility. Even if 19s were a better performance option (they aren't), they would never gain 4 seconds a lap in an apples to apples comparison. I'd be damned impressed if the stock 19s gained 1 second over the stock 18s. And it would almost certainly be attributable to differences in hot tire pressures.