Here is a shot of my test set-up.
Here we can see that the black (dark green) plastic cover was at 136.4°F which is almost identical to the uncovered tire in the original test at 136.1°F so this dark cover is actual heating the tire in this test.
Here we see that black tires and black covers can run almost 40° hotter than when under a white cover. This means the rate of aging of this tire under the dark cover is 12 to 15 times as fast as when a tire kept is cooler under a white cover.
Some will ask if lowering the temperature for a few hours each day is worth the effort, so lets set up a hypothetical situation.
Assume you spend 6 months out of the year in sunny Florida or Arizona or similar location with lots of sunshine. You are parked so one side of the RV is in the full sunshine for 8 hours a day and the sun shines 25 out of each 30 days. Now to make the math easier let’s assume it is always 85°F ambient. Now based on the original test our tires in full sun would be at 132° but if shielded by white covers would only be 96°.
So the question is: How much “older” would the tires without covers or with black covers be than tires that were covered with white covers?
Tire A has a cover. So lets figure that as our control so there are 25 days times 6 months times 8 hours per day of solar heat exposure for a total of 1200 hours.
Tire B has no cover, so it’s temperature is two times 18° hotter. Remember the “aging” rate doubles (times 2) with each 18° increase so this tire spends 25 days times 6 months times 8 hours times 2 times 2 or 4,800 hours.
The difference is 3,600 hours which equals 150 days or almost a half year. In this example each year your uncovered tire “ages” almost one and a half years vs one year for the covered tires. This means that after 6 years of staying only 6 months in a sunny location your uncovered tires have rubber properties that are more like nine year old tires. If we consider tha additional damage done by UV light I think you can see that the uncovered tires will probably have serious sidewall cracking after only 6 years of real time aging.
Covering tires with dark or black covers may shield the UV but they do nothing for the heat and could make the heat worse through radiation.
Bottom Line. If you want to get the most out of your tires you should cover them with a white or light colored cover when the tire will be exposed to extended time in direct sunlight.