Would a 33x 12.5 tire fit on a stock 8 inch wide rim? Im using it as a spare

 

That's pushing the envelope.
Most 12.5 tires want an 8.5" to 11" rim. They prefer 10"
So
You could mount the tire on the rim but unless you confine usage to pavement, you may pop the bead.

 

Ok thanks

 

If you plan on doing any wheeling with your rig you'll want an 8 inch wide rim for a 33/12.5 r15 tire.
A narrower rim helps retain the tires bead in low air pressure situations like rocks, sand, and deep snow.
unless the backspacing will be a problem they should work great.

 

That's all I run and I've run 14.50s on 8s. My 38x12.50s love the 8s.

 

Awesome. Thanks for the inputs guys