Well, looking at KBB retail value, an 02 Miata is about $13k. Private party is obviously cheaper, but since you'd be trading the Mustang in I would think you'd more than likely go through a dealership. Anyway, so $13k + $8k negative equity = $21k loan amount, not including taxes, fees, and such. So lets say $22k for ****s and giggles.

@ 5 years and 6% interest you'd be looking at $425/month. Now that number imo is probably worst case scenario. So, in reality it would probably go down, but is it worth it to save $100-$150 more per month and not have the car you love? Not to mention getting hosed on the trade in?