Fitting a 285 in spare location

Honestly you're overthinking and this is shaping up to be more trouble than it's worth. Especially since you plan to have the spare under the bed where nobody even sees it

 

Have you seen the size of the donuts most vehicles have as spares, the maybe an inch difference between tires isn't going to matter unless you're locked/in 4wd.

 

I think I'd just find a tire 1/2 worn to put under and forget it.

 

The worst part will be swapping the tire out from under the bed every 5k miles. I did 5 tire rotations on my FJ only because the tire was on the rear door and the truck read all 5 sensors. This truck, that damn tire can stay under the bed...not to mention the sensor issue.

 

That's what I did on my old LJ... except no TPMS at all