I use this page a lot. When I see some tires in a metric size, I can look it up on here to see what the equivalent in inches is.
http://www.tirerack.com/tires/tirete....jsp?techid=53
For example LT255/75R17 are around 32.5" in diameter, which in turn is around the same size as a 33X12.50R15LT tire.