This is what I found w/ my tires & rims:
The 295/65 mounted on an 8" rim has a section width right about 12"; it would be closer to 13" if mounted on a 10" rim.

The 275/60 mounted on an 8" rim has a section width right at 11" and it will gain close to an inch in section width spread out on a 10".

IIRC, the "rule of thumb" for section width changes is approx. 1/2" per 1" in rim width.