I just picked up used dounut wheels and tires: a 125-70 15 and a 130-70-16. I don't know what vehicle they came from.

If the lug patterns match and the wheel fits, is there any reason why I couldn't use the 16 or for that matter the 15 as an emergency spare?

 

you should be fine. Just go slow (no faster than 55mph).

if one of them has a 56.1mm diameter center bore, that would be even
better.