McSwell,
A very significant part of your analysis hinges on the cost of gas in the coming years. You state it may go up or down. With the possible exception of a slight drop this winter, gas prices are headed up, up, up. By the time you get your new vehicle up to 200,000 miles in, say, 6 or 7 years, a gallon of gas will be $6-8 if not more.

Not even considering instabilities in the oil market (future Katrinas, Iran, etc.), fossil fuels are a finite resource and at some point--5-10 years, many geologists state--peak oil will occur. Google peak oil for more info or look at the Hirsch report:

http://www.acus.org/docs/051007-Hirs...Production.pdf

As to my MPG, I'm a careful driver and in my hilly area, I'm averaging 53 in warmer temp. and 48 in winter.

Good luck!