What exactly is backspacing? Im gonna get some 31X10.5X15 BFGs on my 15X7 factory rims, how do I ajust the backspacing?

Try this site:
http://www.tirerack.com/about/techcenter.html

BS is a function of the wheel you're using. It is not adjustable.

Backspacing is not something you can adjust on your vehicle. The only way you can change the backspacing is to purchase different sets of wheels.

The backspace is measured by taking a rim and placing it with the outside of the wheel facing down. Then measure from the center of the wheel (where the rotor will touch the wheel) to the top of the rim.

Here is a link with a diagram and description:

Backspacing (http://www.afswheels.com/v2/content/Default.asp?Page=96)

thanks for the info guys, I know it was kind of a stupid question. So will my stock 15"teardrop rims be alright with 31X10.5's?

of course