The dealer called me today, saying I could trade the car in for a new car and not increase my payment; the payment would simply reset for 36 months. I told him I needed to do a little math before calling him back. The math I did was as follows:
Value of Old Car from Kelly Blue Book + PV of 36 months of payments = Cost of New Car.
Cost of New Car - Edmunds Value of New Car = $6000.
Yes, the dealer was trying to fool me into paying $6000 for...nothing. In my particular case, he could not profit from informational asymmetry. But for the person with the average math skills in the US? That might be a different story. We know this selling tactic must work sometimes, because otherwise I would not have gotten the call.
The first lesson from all this is we really need to do a better job teaching math.
The second lesson is that, in the meantime, we need to protect consumers from these sorts of practices.