with 18s i would go with
305/65/18 = 33.6x12/18
325/65/18 = 34.6x12.8/18
but these are a little small
285/65/18 = 32.6x11.2/18
295/65/18 = 33.1x11.6/18
first number, width. second number percentage of width is the side. ex. 65% of 325 = from tread to rim. idk the exact number. and last number rim size. metric is harder to config cause as the rim sizes go up the more percentage you have the more tire you have. thats why some tires that are both around 35" in metric will have 2 different middle numbers cause of the rim size. say 315/75/16 = 34.6x12.4/16 its almost the same as the the 325s but with a smaller rim theres more rubber. make sense? let it sink in for a little bit. but use that site walnuts posted. its the easiest one ive ever used. you can make up tire sizes and it knows what it is. haha
also. not all companys make the same size tire. you might find a size you like but cant find it in the tire you like. so keep that in mind. you might have to change rims or do something else. just ask nvy. his new rims came with his new tires cause they didnt make the size he wanted any more. just a little advice.