First of all, your mustang should be getting much better gas milage than you are getting. Worked on these cars for a lot of years, and to me It sounds like something isn't quite right with the way the car is running. I think It would be cheaper to find out what is wrong with the car, than buy a new car. As for Mitusbushi, you can keep them. From experiance you can keep them. Not a road you want to go down. Take it from someone who works on cars for a living.