re: Is it super dumb to finance a vehicle for 72 months?

In general, I would say less than 3%. But if you have a project going on and you need a car and 6% is the best you can do, then fine...as long as that project yields above 6%.
But car loans for people who just "want a new car" is generally a bad move. Keep your car for another 5 years, take what you would have paid on your note and save/invest it, and in 5 years you'll have a nice nest egg and be able to get off of the endless car note merry go round.