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If you upgrade to the larger 18" wheels your vehicle will ride alot harder. Those vehicles have a harsh ride to start with so it will be worse. If you do get the rims the car will need to be programmed for the larger size. The dealer will need to do the programming and may charge you a half hour labor to do so.
Can I go as wide as 7.5with the 17's? If I do decide to get 18's can I go as wide as 8"