i thought it was rule of thumb to only run with an inch or so of overhang so you don't blow a bead. this is for either street or off road driving.
12.5" of tire on an 8" rim leaves 2" on either side. can't that make it unsafe to drive? if your air pressure gets to low wouldn't it be unsafe? at least with a 10" rim, there is only an inch on either side.