There is a convienent link to right here on the XOC that can help you with that...

http://www.xterraownersclub.com/news.html

Click on Tire/Gear calculators...at the bottom you will see the metric to US Standard conversion.

For your question 265 = Width of the tire at the widest point (not the tread) in millimiters. The second part /70 = the percentage of the width of the tire that is numeric height of the sidewall. The last number is the wheel size (you already got that.

The only problem with doing a strictly mathmatecial conversion is that the sidewalls are never straight (they bow out), to the multiplier is a guide, not a direct size.

Basically the bigger the first number = the wider the tire.
The bigger the second number = taller sidewall (in proportion).

BTW those add up to (approximately) 30.61 inches in height - per the XOC calculator...Close to 31's. As well you could consult the tire manufacturers specifications for their installed height (typically very accurate)

Most are listed here www.tirerack.com

XOC lists my tires at 31.61
YOKO lists them at 31.8



Edited to add:

Quick-draw Ian beat me to it while I was posting the answer.