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[SCCA HP Racer]

Note that the Grand Prix GXP actually has smaller tires on the rear to help quell understeer.

I agree that the Sol is over-tired and I am betting that the tire size chosen was more of a styling decision than engineering. 18 inch wheels are not optimal for performance, they are heavier and have their mass more spread out compared to a 15 inch wheel (slowing acceleration).

In racing we do tune tire pressures. It is amazing how much difference a gain or loss of 1/2 lb in one tire makes, it completely changes the way the car breaks away. The reason for the change is because by changing pressures, you are altering the the spring rate on that wheel. More pressure=higher spring rate.

Bottom line, compound and pressures will make more difference in ultimate handling than small changes in size.