The "fiberglass part"? In that general timeframe and prior, the normal tire was "bias ply", with the cord being the main differentiator among the various tires. The normal tire was a "4-ply" tire, meaning 4 individual cords (plies) in the rubber of the tire for shape and strength. The normal cord material was either rayon or nylon. Rayon being the "smooth ride" cord with Nylon being the one for stength/durability/higher speeds. But the nylon was prone to "flat-spotting" after the car sat overnight and the tires cooled off. Which made the first few miles a bit rougher riding than after they got completely round again. The fix, best of both worlds was "polyester" cord.
As the ply rating was "four", it was discovered that TWO pilies of larger cords could be just a strong and still hit the smooth ride orientation. Plus being a bit less durable, but still "good enough", so "2 Ply-4 Ply Rating" was born.
When better tires were designed for the later 1960s cars, radials had become optional on Ford products (Michelin X). Radials were better than bias plies, but not everybody or all cars could tolerate some of their quirks. So, the cost-effective upgrade was the bias-belted tire. Two radial belts on top of the bias-ply carcass were the result. The better tread wear of the radial but without the harsh ride at lower speeds . . . and they cost less that a normal Michelin radial. The normal belt material was fiberglass, although a few (notably Firestone) had some steel-belted bias ply tires. Fiberglass was the best compromise for cost/tread wear/smooth ride, by observation.
As fuel economy became more important, then "full radial" tires became optional and later standard equipment. As the cost of the vehicles also increased enough to cover the cost of the more expensive tires, too. Radials were also better in handling and stoppiing, too. Plus in wet weather and in snow.
The fiberglass belted tires were good for anything, but steel belted radials could have some poor interactions with Unibody cars in certain speed ranges. Impact harshness and road noise, notably. Fiberglass belted bias ply tires did not have those issues. As all Chrysler products were UniBody, the fiberglass belted tires were a good "fit" for them and their customers, by observation.
Enjoy!
CBODY67
