What type of wheels are you talking about... The actual car does not have backspacing.
Maybe you are asking what the most common backspacing is for Weld wheels? In that case it is 5.5" for 15x8 wheels and 4.5" for 15x7. Some people use 4.5" on 8" wheels but they will stick out a little more. I use a 5.5" backspacing with 15x8 wheels. Check it out
Wheels