Ben, I took it as a given that I would need to invest the money from the 3.9% interest loan into something that would yield more than a 3.9% return. What I don't follow about your idea of making money from a car loan is the "CAR" part. Last I checked, concerning car loans, the only thing a bank will allow you to invest in with that money is an automobile, which starts depreciating before you even drive it off the lot. I suppose if you got a lender to just give you the cash so you could invest it in something other than a car, then you could make money on that investment. But it wouldn't be a "car loan" anymore...
Money is fungible. It doesn't really matter (I don't think) whether it comes from a car loan or in a 3.9% loan from your Aunt Tillie. If you can invest it at 4.9% or 5.9% you are well ahead of the game. Right?