The senators want the loans used 'to pay for the construction of new cars, trucks and SUVs that are 25 percent more efficient than the CAFE (or corporate average fuel economy) standard for that vehicle in the year the retooled auto manufacturing factory reopens,' they wrote.

The current interim ruling, issued by the department in November, defined the 'base year' of this program as 2005, instead of the year a retooled factory reopens. This is significant because using the 2005 baseline fails to include higher fuel efficiency standards.

Some auto executives predict the department will issue loans by June 30.

'Providing these firms with a precise timeline would allow them to plan their business operations during a time of extraordinary financial challenge,' the senators wrote.

More than 75 companies have applied for more than $34 billion, including Detroit's Big Three, which have sought about $22 billion. California-based electric vehicle startup Tesla Motors Inc. wants $400 million and Troy-based auto supplier Delphi Corp. has sought undisclosed loans.

General Motors Corp. had sought about $8 billion for use in its plug-in electric Chevrolet Volt and its Chevrolet Cruze programs. Ford Motor Co. has sought at least $5 billion by 2011. Chrysler LLC has sought $8.5 billion from the retooling program.