18's won't fit on an S13 without fender rolling and They will be hard to fit on an S14, you'll need to figure out the offset. BTW, the width should be half or more then half of the diameter. SO 17's on front you need at least 8.5, let's say 8 cause more prolly won't fit without spacers, in back with 17's you need at least 9 to not be gay.