According to reports across the US media, each recipient now faces a tax bill of around $7,000 (£3,890) if they keep their car, which comes with a showroom price tag of $28,000, because the US taxman counts the automotive windfall as income.
Harpo Productions, which produces the Oprah Winfrey Show, said the same tax rule applies to gameshow prizes.
Some of the contestants who have spoken to the US media say they were already aware of the tax liability.
William Toebe, from Wisconsin, told the Chicago Sun-Times that his smile was forced, because "paying the taxes was the first thing that popped into my head".
The publicity stunt for the car is one of the biggest television giveaways to date, with a retail value of $9.7m, and is understood to have come as a result of a chance airport lounge meeting between Winfrey's friend Gayle King and Larry Woodward, chief executive of GM's advertising agency Publicis Groupe-owned Vigilante of New York.
However, the almost $10m cost of the cars is nothing compared with the media coverage the huge motor giveaway has generated.
The guests on the special giveaway show, which was the first of its new season, were selected from people who had told the programme of their need for new cars.
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