235/45R18 is too tall - stock is 225/40R18, you don't want to go with a higher aspect ratio than that. 225 * .4 = 3.5" sidewalls. You want to stay as close as possible to that sidewall height. 235 * .45 = 4.2" which is much too tall, you will throw off the speedometer and possibly rub. 235 * .4 = 3.7" which is more acceptable.
Proper size is 235/40R18, if you want to go wider.
http://www.miata.net/garage/tirecalc.html is a great resource for seeing size differences.
what are you talking about? who said anything about 235/45?
i said the wheels offset is 45... ET45 that has nothing to do w/ tires