In the mid to late 80's, IROC-Zs with the 16" wheel option had a factory wheel spacer for the rears. Why they did this I will never understand as the wheels already had different offsets front and rear.

Does anyone have a set to measure the thickness for me? I am looking for something around a 1/4", but do not remember if those were that thick or not.

 

I was just researching this topic, RKrause has an interesting thread in the Advanced Discussion... you might find it interesting. Good luck.