Utilitarianism discussion

The concept put forth by John Stuart Mill simplistically does or does not explain day to day behaviour?

It seems to me that although the theory does fall short in a number of situations when aulturist behaviour is required, in most day to day situations utilitarianism does explain why people behave as they do.

This is my 2nd reading of Mill's book after a very long time, and now I would modify my question to remove the simplistic from the question. I made my comment for discussion based on my memory from my first reading.

I am wondering about the discussion of 'expediency' by Mill, (page 22 of my version). I had not realized that expediency had the meaning of convenient rather than what is morally correct. (I just looked it up in the dictionary) I think I have always used it meaning the quickest way to accomplish some thing and did not mean anything about morally correct. Does anyone know if there is an older meaning for this word because that seems to be the arguement Mills is making or am I incorrect?