First off we have a Metric to Standard Tire Size Conversion site. I personally like this site because not only does it convert tire sizes both ways but also allows you to find an equivalent size if your changing the size of your wheel. On top of that it then gives you list of some tires available in that size too!
TSC Metric Tire Conversion
Another cool converter is this one. It allows you to convert the offset on wheels to backspacing, and the other way around too. Pretty handy when your looking at putting on that new lift kit that calls for 4.5" of backspacing but all the wheels your looking at only list the offset such as 12, 0 ,-12, etc.
MRW Wheels Offset Chart
Here's some forums I like to frequent that I've learned a good deal of information from.
