You get half a year's use out of each set of tyres so they last twice as long in total, but after the fourth year their material will be inferior compared to new, even if the car was not driven much.
Let me provide a look at how a car company tracks profitability of a product program: measured are material cost and labor, and these are deducted from the selling price.
Back in the Greatest Generation years, the American Dream was to own a car and a house and have a life long job that would allow you to afford those material goals.
German carmaker BMW - SGL Automotive's only customer and owner of 49% of its shares - uses the material to produce body parts for the main cabins of its futuristic plug-in models i3 Megacity and i8 sports car, which will hit the road in 2013 and 2014.