The trim rings are the correct width @ 2 3/4 just like a factory 15x7 trim ring so let's do the math.
If you have 4.5 backspace and 2 3/4 trim ring your at 7 1/4.
You say there is an additional 1.5 gap right? 7 1/4 + 1.5 =8 3/4???

How is that possible? I think the backspace on the wheel is wrong or the trim ring isn't going all the way on.

Like I said earlier in this post a factory 15x7 has 4 1/4 backspace, trim ring is 2 3/4 4 1/4 + 2 3/4= 7"

Your rim has to have 3 3/4 backspace no???? 8"- 2 3/4 = 5 1/4 - 3/4" ( the 3/4" is the gap you should have been left with)=4.5" backspace.

8" wheel with 3 3/4 backspace leaves 4 1/4 to front so then minus 2 3/4 ring which = 1.5" left for the gap.

Someone please correct me if I'm wrong in my thinking.
Ron