[swierker]

Offset and backspacing are closely related. If you know one you can get the value of the other. There are many calculators out there. Here are a couple I've been using:

http://www.1010tires.com/TireSizeCalculator.asp

http://www.rimsntires.com/rt_specs.jsp

http://autolounge.net/calculators/tirespacing.html