The reason you would choose 18" wheels over 17" would be to run a lower profile tire,
which would look, and all other things being equal, have better cornering response.
On the other hand they may be heavier, and reduce acceleration, and braking slightly.
Also, it depends on what size of a car you have, a Nissan 350Z for example would be better suited for bigger wheels, whereas a Honda Civic may be better suited with 17's.
Chat with our AI personalities