Backspacing is less on 15x7, it is 5.25”. However, I am not using a 15x7 spare at the moment, I’m using a 15x6 which is also 5.25”, but the tire bulge from the narrow rim is why the 1.22” bumpers are used.
30x9.5 on the 15x7 wheel is not snug enough on the 1.22” bumpers, and it is way too tight...