What about the 1.2 million tires on the 300, 000 cars that are in the Hertz rental car fleet at any one time?

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Additionally, the U.S. car rental business came under pressure from fleet constraints and associated efforts by rental car companies to drive up pricing.

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Yes, the company's North American market share is down, but that was deliberate--Ford's CEO decided not to aggressively pursue virtually profitless sales to the rental-car and government-fleet vehicle markets.

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Excluding sales to fleet buyers such as rental car companies, Honda is No. 1, said Mike Acavitti, senior vice president of marketing for American Honda.

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Spokesman Nick Richards blames the slow growth on a cut in low-profit sales to fleet buyers such as rental car companies as GM tries to boost the brand's resale values.

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Certainly, it seems better to have one fleet with two potential outlets for it (both the regular rental and the car sharing) than it does to have two separate fleets each with their own specific downtime.

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Zipcar has thrown a monkey wrench in the traditional car rental business over the last few years, and it currently serves over 673K members with a fleet of over 9K vehicles.

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