You probably have lighter rims and tires with the 17" setup than your 18's.Nothing extraordinary, but damn..the ride quality is waaaay better! And I think they are a bit over inflated by the seller but it's day and night difference, like a different car. Also went with different tire brand other than Kuhmo or Falken. These things are staying on!
I also believe that Hyundai didn't bother to calibrate the suspension for the turbo versions with the 18", they are basically using the NA calibration, because these 17" wheels/tires feel like they were made for the turbo far as handling and ride quality.
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Less unsprung weight can make the car feel completely different and lighter on its feet. 😉
Btw those rims look sweet on the car and compliment the deep blue wonderfully. 😍