I believe you are referring to outer diameter of the 15 inch wheel/tire in comparison to the outer diameter of the 16 inch wheel/tire, and that is correct, thanks for the observation. On the other hand, these bunch of old tires will find their way to the recycler once the time for new ones arrives. At this time the focus was to fix things until I got a chance to do bodywork, and interior, and seats and, and, and... My goal "was" to drop money on wheels and tires only at the very end, but it happened to happen before, LOL.
As for backspace, it is the distance from the wheel mounting surface to the edge of the rim. It is a more direct and thus more interesting measurement of the wheel offset. And it is actually the info that manufactures stamp on the wheel instead of offset. I was looking for similar backspace to the OEM wheels to keep them inside the fenders to the same amount.