Originally Posted by mathjak
What about the ones on the vehicle?. they are all exposed to the elements equally
Tire covers, also called tire guards or tire protectors, slip over wheels to shield them from weather and harmful ultra-violet (UV) rays that can prematurely age rubber. Ungaraged vehicles, especially those that get driven less often, are at higher risk to develop cracked sidewalls from sitting in the sun. Tire covers can protect your investment by blocking UV and retarding the aging process.
There is no greater enemy of rubber than UV and ozone. To combat these environmental effects, tires are made with a "competitive absorber" known as carbon black. This compound is what gives tires their color. The molecules in carbon black absorb UV rays converting them to heat that can dissipate off the tire. Over time, however, carbon black becomes depleted and the rubber turns gray and brittle. By shading tires with UV-blocking covers, you are effectively extending the life of tires.
Tires that sit for long periods are even more susceptible to UV damage because driving actually helps tires remain supple. When a tire is driven the flexing of the rubber causes it to become heated, stretched and worked; a “kneading” process that stirs protective wax compounds that rise to the surface, keeping the rubber protected and in good shape. Without flexing, tires become prime candidates for premature breakdown. Add constant exposure to UV rays and the aging process is accelerated.