http://www.roadandtrack.com/special-...transformation
Short version: Even with lighter-than-stock 18" wheels, the larger diameter puts more weight farther from the wheels' axis of rotation, making for slower acceleration and a lower top speed. And simply replacing the Prius-sourced stock tires with a set of stickier rubber (still in stock size) dropped lap times by 2.3 seconds. You don't need huge tires on this car- they reduce steering response and acceleration and top speed. Add 100 horsepower and the story may be different.