[JRTJH]

Remember that part of the "preservation/aging process" for tires is being rolled under weight of the trailer, to release the oils in the rubber. Without that, tires "age faster". When you add sitting on the rear bumper under a black, gray or even a white cover, your spare will "get old before its time"... So, realize that the tires "on the trailer" will lose about 10% of capacity per year and "time out" at about 5 years, the spare, sitting on the bumper will probably lose capacity faster and age faster, approaching 15% or even more per year. So, if that spare has been on the bumper for 2 years already, that's close to the same as 3 (or more) years on the ground. Don't ignore your spare by thinking it's going to "just be there, stagnant" until you need it. Watch it closely, when you get to 3 years from the manufacturing date, start considering whether you want to risk using it, even as an emergency measure to get you to the next tire dealership.