I drive 5K each year on my winter car, a 91 Grand Marquis that gets 17-22 mpg. It's condition - both mechanical and visual - rivals many newer cars. The market value of this car is very limited, regardless of how perfect it may be. I'm not going to throw away a good and elegant piece of iron (and comfort) and shell out big money just to save a few hundred dollars in gasoline.
Assuming 5K driven per year, $3.20 per gal, 20 mpg with the 91GrMarq, and 32 mpg with any new car, I stand to save $300 per year. Big whoop. Not worth it in my case.