I believe the manual says within 5%, but I would have to go back to check.
Usually if you air it down, you can put it in. I have 265/70s, so I'm in the same boat. Usually I just leave mine in the garage. I do carry plugs and a compressor thought to get off the trail. Worse case, I'll call someone to go grab my spare and bring it to me. If I'm going out of town, I'll throw it up top, or in the back depending on how much space I have.I have four LT265/65R17 tires, but my P245/65R17 spare is still inside the compartment untouched. The new tire size won't fit in there and I know that I could put the larger tire on the Rhino Rack roof platform, but choose not to.
My 'plan' is to limp to a nearby location where I can get the proper size tire changed over. I've actually been thinking about just buying the LT replacement and just leave it in the garage until needed, but spending 4 months in one location and 8 in another may make that idea less than ideal. I could order the LT version and typically get it delivered in a few days, worst-case. There doesn't seem to be a perfect solution, unless that is it.