Originally Posted by Radiation Joe
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.
If you read the thread I referenced it had all the details - not just the wheel size. 18's were 275/35/18 fronts, 285/35/18 rears. 19's were 275/30/19 fronts, 295/30/19 rears. I'd been running with the 18's on two other tracks beforehand and I ran them hard, pushing to the limits, learning the optimum temps that they liked. Then I switched to the 19's for the first time and rocked my lap time. I myself was shocked to say the least. The car felt very different. The wheel size of the 18's were 18x10 all around and they weighed 20 pounds each. The 19's were zcp's that were 19x9 fronts and 19x10 rears that weighed about 26 pounds each. Sure the wheels are a bit smaler for the rubber used on the 19's but gave the car so much more grip. I noticed on the PSS's the 19 rubbers sat much more square than the 18's which actually had a lot more stretch to them.
As you said there are lots of variables to consider but bottom line I was able to turn faster times at road atlanta with 19's even with greater rotation mass.