do u have one under there right now/ what size is it? take that size, convert into inches than take a tape measure, crawl under there and add half of the difference to the tire all the way around, for example if your spare is a 31, add 2 inches all the way around the tire. I am going to say that it will be close, my spare is a 33 and there isnt alot of extra room
The tread is worn down a bit on the spare so I wasn't sure how much to allow for it, and it's a 245/75 16. I'll measure the whole area, as you suggested, and post it for anyone else wondering. worst case, I'll one of the 285's or 265's as the spare (truck had smaller tires in front when I bought it).