President Bush on Tuesday signed into law a low-interest loan package to aid U.S. auto makers, but those struggling companies will still have to wait months to find out how and when they can tap the $25 billion designated to smooth their transition to building more fuel-efficient vehicles.

The loan package was approved last year as a way to help auto makers and their suppliers meet fuel-economy standards set by the federal government. But the funding for the package wasn't passed by Congress until this year. One estimate put...