My car had also a Firestone spare tyre 800-15. It was a
puncture safe tyre which was new to me.I don`t know how
well they worked but I understand it as first there were pressure
on both sides of the half innertyre.When you got a puncture the
air swooshed away from outside of the innertyre which made
the pressure difference close the rubber valve of the innertyre.
So it left some air in the tyre but how much?
The tyre is thrown away so I don`t know if it was a Supreme
but I had to save the inner tyre.