dont believe time value of money makes it worth the extra monthly payment
You're going to the trouble of figuring out the $s of interest the extra payment will save, but you are willing to "believe" it is not worth it without figuring out how much money you'll make putting that $50 in savings, mutual funds, CD, etc.?
That $140 is real, but the principal is real, too. If you don't put the $50 towards the car, what are you spending it on? Pizza? Call of Duty? Drugs?
This analysis seems penny wise and pound foolish.