Another Car Puzzle

http://commons.wikimedia.org/wiki/File:Rumble_Seat_(PSF).png

A puzzle from Oswald Jacoby’s Mathematics for Pleasure (1962):

The MacDonalds are planning a long car journey of 27,000 miles. If they use tires that last 12,000 miles each, how many tires will they need, and how can they make the best use of them?

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