Rear end ratio
I have a '77 and I have always wondered the same thing .
According to conceptcarz.com it was 2.50:1. (Definitely Too High) It doesn't say anything about being engine specific so it might be the same for all the engines of that year.
Search Ford Thunderbird Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Thunderbird Experts