Feif Un:R’ Tam’s zman was based on where he lived, in France.
Rabbi Ovadia Yosef proves that this is untrue based on a couple of factors.
A) Rabbenu Tam answers a stira of R’ Yehuda (who he paskened like) and mentions: “and for sure it is not vadai night for R’ Yehuda until after 4 milin (each mil is 18 minutes) from the time of shkia. And R’ Yehuda lived in E”Y as it says in shas many times, “Tana Be E”Y Kai”. It would be a strech to say that R’ Yehuda was talking about France in Pesachim 94a.
B) Even though the S”A lived in E”Y, he still paskened like Rabbenu Tam who lived in France. Which is interesting, because Maran saw with his own eyes exactly how fast the Shkia is in Israel and was still steadfast to the Rabbenu Tam shitta.