Can any one exptain to me what back spacing means

 

Backspacing is the distance from the axle pad (the part that contacts the wheel hub) to the inner edge plane (inboard flange).

Deeper backspacing moves the wheel and tire inboard on the vehicle; shallower moves them outboard.

 

ok thank you do you know how far a 3.75 and a 2.5 back spacings would push out a 8 inch wide rim

 

well stock backspacing is around 4.5"

if my numbers are right if you did a 8 in wide rim thats 1 inch over stock(.5 on each side)
3.75"bs would be .75" over stock.

.5 + .75 = .8" or almost close to a inch more sticking out... or something like that.

if you want to get an idea you can search for pics of different BS mounted up

 

ok thank you that helped me make up my mind