Why would that hurt an open diff?

 

it causeses constant wear on the diff as if the car where constantly turning it may not break it necessarily but that still not something you want to chance.

 

Yeah...I can see that. I wouldn't want to do it, either, unless it was an emergency. I have a spare that is the same nominal size as my bigger set of tires (275/65R8) but it's more worn out and not the same brand....so it may be .3-.5 inches smaller. I would run that if I had to.