Hi there, i'm new to this site, and i see this post is old but though i'd share my 2¢ anyways... I have been a Jeep owner and car buff for years. In my opinion the best tire/wheel calculator online is http://www.rimsntires.com/specs.jsp
tire-size-conversion.com has a good metric to inches and inches to metric. it also has a speedometer calibration one too. type in your stock size and your new size along with a speed and it tells you how fast your actually going at that speed.