Found these surfing around and thought some folks might want to use them. The first link only does metric calculations but provides offset and backspace info.

http://www.bigcustomwheels.com/rt_sp...B097B01DF8ADA9

The second link is a downloadable program that works pretty good. It provides the basics and does its calculations for metric, LT, and inch tires for the die hard off roaders.

http://cat.cc/tiresize.htm

Keep in mind that these are computer calculations that will get you in the ballpark and shouldn't be taken as gospel. So ask advice.