[dBlast]

18s on the left, 17s on the right.

If the tires are sized properly, they should have the identical overall diameter as the stock setup to maintain the correct speedometer reading. If you choose the same offset wheel and a 205mm wide tire, then the space the the tire/wheel combo take up in the wheel well will be exactly identical to the stock setup. A 215mm wide tire should also fit.

The critical point here is the offset (the vertical relationship between the outer edge of the wheel and the point where the wheel is connected to the hub.)

The difference between the 18 and 17 inch diamter wheel is then the tire sidewall height. The shorter tire sidewall height used on the 18 inch wheel will produce less side to side deflection but will give you a much harsher ride.

The difference in the above cars is noted when the red car has to negotiate a railroad crossing at a crawl and the blue car has to only slow slightly to pass.