The way I work to figure out ideal size tire for a given wheel width is to start from 7.5" rim = 225mm wide tire. Move a step down to 7" rim = 215 tire, and 8"rim = 235 tire... similarly our 6" stock wheels have a 195 tire and a 9" rim would ideally have a 255mm tire.
i.e. for every 0.5" step, add or subtract 10mm from the tire width. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>