[ToyoForLife]

a hard cover is preferable, but it just needs to be covered to protect from the sun. Tires have a shelf life and aging is one thing, aging while being exposed to UV is another. The tires on the ground may get hit by the sun, but they do get shaded by the car itself and it rotates whereas your spare stays in the same position all the time.

 

[NY-TJ]

a hard cover is preferable, but it just needs to be covered to protect from the sun. Tires have a shelf life and aging is one thing, aging while being exposed to UV is another. The tires on the ground may get hit by the sun, but they do get shaded by the car itself and it rotates whereas your spare stays in the same position all the time.

part of regular servicing is checking the spare tire for pressure, aging, etc.
most owners manuals I have seen mention you should check the spare tire when checking regular tires
as long as you do that with keeping it protected there's not much to worry about.

 

[RoughDay]

It really isn't that big of a deal to have the spare tire exposed in the sun. Look at Wranglers... always just out in the sun and it's never been much of an issue for those guys.