I don't know to which GoodYear 'Diamond Treads' you refer.
The original design from the Ford Factory (and Dealers) of 1930 and 1931 have been reproduced and are available from most of the Model A Parts houses and other sources.
The later 'After Market' Goodyear 'Diamond Treads' of the 1940's and '50's, which have a different side wall design (vertical grooves), are indeed hard to find today, and do represent possible 'aging' issues. As has been said, with proper storage, these tires (and almost all tires) can last for decades and still be safe to use. Sun light and heat are the main elements that degrade rubber tires. Ofcourse there will always be a chorus of 'nay sayers', who will be quick to contradict this sentiment, and I am sure they will chime in here.
If you share these doubter's concerns, then the uncontroversial choice would be the NEW GoodYear Diamond treads. Plus they have the great benefit of being currently available, whereas the old 1940's /''50's design are very difficult to find - especially in safe usable condition.
I do have a set of these Goodyear Diamond tread tires from the 1940's (with red dots) and they are in excellent and safe condition. As has been mentioned, while these do have a genuine 'Diamond Tread', the sidewall is noticeably different from the original Ford Factory tire.
- Doug Vieyra, Eureka, Calif