15x12 wheels?

Has the rear axle been narrowed or the wheel wells modified? You also want to do a baseline check over on the opposite side with a wheel and check dimensions. If the axle has been modified it isn’t uncommon to have one side offset some. I would imagine 295/50-15 would be a safe size to run without issues. Maybe a 60 series. If the rear has been lowered, that may be the largest that fit, while at stock ride height 315/60-15 fit with sometimes, but not always, light metal trimming.