Even the automakers don't know the exact environmental cost per car:

http://www.nist.gov/mel/msid/dpg/sccwmp.cfm

However, estimates for CO2 emissions related to the manufacture of a typical car, including parts suppliers and transport, range from 10% to nearly 30% of the total CO2 emissions over the car's lifespan, which is a sizable up-front environmental cost.

http://environment.about.com/od/environmentfriendlyautos/a/new_old_cars.htm
http://www.ae-plus.com/Key%20topics/kt-emissions-news4.htm

Adding in the natural resources needed to build a car and their extraction and refining costs, plus the environmental costs of trashing the old car (even if much of it can be recycled), and it makes more sense to maintain the old car, or maybe replace it with a used car with better fuel economy.

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