Neither was ozone factored in.
"Unsafe At Any Speed" The biggest joke the precursor of NHTSA ever played on the gullible mantra-soaked US public. This is a political not factual US government organ. Like the NWS
Deutschland Uber Alles for Volkswagen and Porsche. Ooooooo and for the Francois Renault Dauphine.
Today's tires contain a MINIMAL amount of authentic rubber. It is now scarce and costly.
About forty years have passed since I climbed the stairs ina friend's tire warehouse and toppled a stack of "new" 1000 X 20" tires sitting against a 10 horsepower motor driving a horizontal air compressor. Every last tire had micro cracks around the bead.
Plastics and elastics containing CARBON BLACK are more O3 resistant than identical carbon products sans carbon black. This is why BLACK plastic tie wraps live in sunlight while natural white ties become brittle.