In my first read, I didn't catch the part (that you mentioned) about missing. Whatever the cause (many possibilities) of it, this in turn will be a major cause of dropped fuel economy, as you are clearly wasting some fuel as this occurs. So, assuming you can correct the cause, you can look forward to an improved fuel economy.

And, 58,000 miles on a 1980 -- congratulations on such a find!

Happy New Year.