Ok I'm trying to figure what backspacing I want on my rims. I am gonna ask for them for Christmas so I want to get going on it to let my mom know in detail what I want. I'm gonna get a set from BlueTorch Fabworks, $150 a rim for 15x10 beadlocks with any backspacing. So how can I decide what backspacing I want? 4.5 seems common, any reason?