GM executives say $4 billion of that would need to flow to GM by the end of this month to keep the company in business.
The $18 billion figure is $6 billion more than anticipated for the Detroit automaker. Ford and Chrysler have yet to say how much money they would need to keep operations going.
The Big Three came to Washington two weeks ago to make their case for a combined subsidy of $25 million.
Ford is now asking for $9 billion in loans. Chrysler, the only private company among the three, has said it needs $7 billion. If each of the companies received their requested funds, the auto bailout would cost taxpayers $34 billion.