275 on 8x15

The checking rim for a 275 is 275/25.4 x .7=7.58.

The problem with this is; it's a checking rim. If you install a 275 on a 7.5 it will measure 275mm, that's all it means. If you install a 275 on a 10 inch rim, it will not measure 275 anymore, perhaps closer to 300.

If you install a tire onto too-narrow a rim, the air pressure inside it will tend to pull the edges of the tiretread up off the road. And thus the treadwidth will not ALL be on the road...... which was the reason for installing the wide tires in the first place....right? So the solution might be to reduce the tire-pressure. And if the pressure has to be reduced too far, the car handles like crap in anything but a straightline.
Having said that, I ran 275/60s on 8.5s at a slightly reduced pressure, 28IIRC,and they aged well. The 50series not so much.
I imagine 275/60s on 8s will want about 24 to 27 psi, depending on the weight above them.
At 24psi, cornering gets frisky, watch for the rollover, don't panic...