Fewer prospective new car buyers are considering General Motors and Chrysler vehicles as consumer attention focuses on their calls for further US government aid, a survey showed.

The proportion who said their primary choice would be a vehicle from one of the GM brands dropped by almost 12%, while the share for Chrysler dropped by a third, according to a CNW Research survey cited by Reuters.

"The decisions to cut back on advertising haven't helped GM or Chrysler. Nor has the talk of bankruptcy, which continues to haunt those brands," CNW said.

Ford is the only US automaker not to have accepted emergency government aid and saw a nearly 12% increase in the share of car shoppers who said it would be their first choice.

The news cames as the co-leader of the Obama administration's auto task force said GM and Chrysler might need "considerably'' more than the $US21.6bn they're seeking to stay in business, according to Bloomberg News on Friday.

"It could be considerably higher; I won't deny that," Steven Rattner said.

"What they've asked for depends on them achieving plans that are somewhat ambitious," he was quoted as saying. "Like all management teams, they tend to take a reasonably, slightly perhaps, optimistic view of their business. So it could be more. I can't rule that out."