Hello, my Grandsons girlfriend had three different tire sizes on her car, the front had two totally different tires sizes, I understand that that is not good, so I took her to get a new set of tires. The tires are 17", but when I looked at her spare it was 16". I would think it would be really not good, to put it on if she got a flat, but I really don't know what would happen, as I am not a car expert. I haven't been able to find anything online about anyone doing this. Just curious what issues this would cause.