The finance arms of General Motors and Ford are both the most profitable divisions of the two carmakers and the most exposed to the extra unsecured borrowing costs Thursday's downgrade to junk bond status brings.

Eric Feldstein, chief executive of GM Acceptance Corp, warned earlier this year that a downgrade - which happened on Thursday - would cause "a lot of turmoil" in the company's bonds. He said it could prompt spreads over government debt to more than double to 600 basis points for a while.