i think 18's are way too big for this car im sure you'll come across issues with the wheels rubbing especially when turning. seen a spark in person at the auto show and it's wheel wells don't look like they can take a wheel that big.
I guess the 18's are a way big for the rides to be fitting onto such sort of ride. Seen an ignite in individual at the automatic display and it's rim bore holes don't look like they can take a rim that big.
I don't think 18's will work, but I may be wrong. Wouldn't the speedometer and shift points (if auto trans) be incorrect when running such large wheels? Or can the Spark's computer be calibrated by the dealer to account for different wheel/tire sizes?