Any pics? I have been considering this tire size. Thanks!
Here are BFG M/T's 255/85:
285 vs 255 height difference

Side view

Stance

The 255/85/16 and 285/75/16 are usually the same diameter. The 255 is just narrower in width, which is why it was suggested for a spare. It would not hang down as low as the wider 285 tire in the stock spare location.
Completely incorrect!! When searching for my new tires, I looked at several different tires, in both 285 and 255. They ranged from 32.3" to 33.4", that's a full inch. As you can see in one of my shots, the 255 is 1.5" taller than the 285. You will need to check specs and/or measure each different tire size, by each different manufacturer to get an accurate and true diameter.