(in reply to your erroneous post in the General section): Ya know, wheels aren't unique to the Xterra, you can read all about what offset, backspacing, and all the other parameters are on any number of sites. Tirerack.com has a decent little info page, and of course Google will find most anything.
Edit: hell, here ya go...
http://www.usacomp.com/Offset.htm
The X wheels, at least the 7" wide 15" and 16" diameter ones, have 40mm positive offset. Not sure about the 8" wide 17s.
Brent