Backspacing questions?

[03duramaxxxx]

I have a 2003 duramax with stock wheels and tires. Im going to go with a leveling kit then some kmc xd 20's with 285/50/20 nitto terra grapplers. I was reading about this backspacing thing with bigger rims could anyone explain to me what this is and do i need it? ive heard 4.5 inch backspacing in the rear with 20s what about the front do you just leave it? Any information would be helpful.

[DunesRunner08]

4.5" backspace will push the tires out past the fenders a little... you can run up to a 6" with 20" or bigger (depending on the wheel).... Im not a fan of the tucked looked, I like the tires to stock out a little from the fender... so the 4.5" BS would work just fine in front and rear... If you ever lift the truck, you need to go with a 4.5" to clear the spindle, but you dont have to worry about that with just a leveling kit