The only things I could find in web searches indicates that your Firestone bias ply tires are manufactured and sold with the understanding that they are for use primarily as display show pieces on restored or vintage vehicles, with limited driving usage at highway speeds of 65mph and less.
Dan, if you come across that information again, could you post it? I couldn't find anything online, so I called Coker and talked to a rep since they sell the Firestones. Take everything with a grain of salt, as we all know what "customer service" has become these days, but here is what he said. Take it for what it's worth, and decide for yourself whether it's good information or not. Also, to be clear, this isn't ME saying this, it's him, and I'm in no way posting this in an effort to promote, defend, or uphold what he said. I'm seeking answers myself, not an argument!
So that all being said, the rep claims that even though there are no official speed ratings for the bias ply tires, the DOT requires all on-road tires, regardless of it's construction, to be at least "Passenger", or "P" rated, which he says is 93 mph. I then said just to be clear, you're saying I can safely travel at 65mpg on these bias ply tires without fear of a failure due to more speed than the tire is capable of handling. And his response was "oh yes, absolutely".
I'll say again...HE said that, and I'm in no way trying to defend his words, or argue in any way. I'm just adding what he said to the conversation, and everyone can come to their own conclusions. For me personally, it makes me feel slightly better, but not much I will look for further confirmation.
