54.5 MPG by 2025

One notable safeguard is the 'midpoint review' that includes evaluation of the new technology and how well they are received in the marketplace.

Environmentalists and auto manufacturers agree on the technology advancements making the tough standards achievable.

"These proposed standards can be met using well-known technologies such as better engines, lower-cost hybrids and electric cars," said Roland Hwang, transportation program director at the Natural Resources Defense Council.

"The standards are going to be quite stringent and a challenge," said Scott Becker, a senior vice president in the United States for the Japanese automaker Nissan. "But given the range of technologies that we either have currently or are developing, we will be in a position to meet them."

"The automakers are confident that they can achieve incremental goals each year, but the real test will be if costs can be lowered enough so consumers will want to buy more electric and hybrid models."

Thanks to John Hartz