An issue not considered often with our babied collector cars is the condition of those special tires fitted to those factory or period correct rims. Much as a travel trailer spends most of it’s time sitting, our cars do the same. Some of us are fortunate enough to have space in a covered garage for this; others must get by with expensive car covers outdoors in the elements. Either way, nature’s elements are wreaking havoc on those tires.

Ozone is a naturally occurring gas in the atmosphere that promotes oxidation of rubber. It’s given off by running electric motors as well, so those cars stored in areas shared with pumps and compressors are subjected to this gas regularly. The tire’s age is limited by the oxidation that the carcass can sustain and remain safely pliable. Obviously, none of us would run tires that are visibly cracked or dry rotted unless we’re gleefully navigating our way home with a once-in-a-lifetime buy of a pristine $500 GT 500 from a farmer’s barn! Tires that far gone are easy to recognize, but that damage is present even without those telltale checks and cracks in the sidewalls.

Rubber hardens as it oxidizes, and it turns out that the useful life of a tire (mounted or on the shelf) is approximately 5-6 years. This is in line with a rule-of-thumb used by the recreational vehicle industry. The average age of tires replaced in that industry is 7 years. Apparently, there’s a little Scott in all of us! It appears that our glamour tires have a life limit; the same applies to performance tires as well. With a hardened composition, that custom-oversized, soft-grip performance radial won’t pass the skidpad test any better than your grandfather’s Airstream trailer tire.

Frequent use will keep the surface oxidation scrubbed off the tire as well as work the softening chemicals in the deep rubber to the surface. The tires will stay more pliable longer with frequent use when compared to tires even carefully stored in controlled conditions.

Therein lies the moral: no matter what the source of the tire or to a lesser extent, the care given to it, all rubber tires change composition gradually. Eventually, the change is drastic enough that the tire’s pliability is compromised and is of questionable safety. All this can be without any obvious signs of decay on the tire sidewall; signs with which we are all familiar. Knowing the manufacture date of your tires will help you track when you approach the life limit of the rubber mounted on your prize.

I’d like to credit Tom Wilson, editor of the Technical Correspondence column in Road & Track magazine for the majority of the information presented here.