Update: The choice of Earth Day for the announcement of new CAFE standards is no coincidence. Leapfrogging the timeline established by the original law, the the Department of Transportion's proposal calls for a jump in fuel efficiency for all passenger vehicles to an average of 31.6mpg by 2015.

U.S. Transportation Secretary Mary Peters described the plan as "historically ambitious, yet achievable." Ambitious indeed, the new regulations call for average passenger car fuel economy of 35.7mpg and average pickup truck and SUV economy of 28.6mpg.

The plan is designed to phase in over the intervening time, with improvements required every model year. The biggest increases are near the beginning of the plan, with average fuel economy set to reach 30.5 mpg by 2013, leaving only 1mpg improvement for the final two years of the plan.

One of the most insidious portions of the plan, however is its 'footprint'-based economy requirements. Large makers like Chrysler, GM, Ford and Toyota will only be required to average between 33.6mpg and 35.5mpg while small-volume maker Porsche will be required to achieve an average fuel efficiency of 41.3mpgby 2015, based on current product plans, reports The Detroit News.

The auto makers themselves are offering restrained commentary on the bill, with even Honda calling the goals "difficult" although they can be achieved. Ford, GM and Toyota are equally committed to meeting the requirements, and acknowledge the challenge the standards present.

Original: Everybody knows CAFE regulations will force carmakers in the U.S. to lift their fleet-wide average fuel economy levels to 35mpg for both cars and trucks by 2020, but what the feds haven’t outlined is what interim fuel economy targets will come into effect in the meantime.

Last month the National Highway Traffic Safety Administration (NHTSA) submitted its proposal for annual fuel economy increases to the White House and now there are reports claiming the government will announce the final interim standards at this week's Earth Day event.

Although no official details have been released, Automotive News reports that the Earth Day announcement will reveal proposed fuel economy standards for cars and trucks in the 2011-15 model years. The Department of Transportation has also revealed that Secretary Mary Peters will also make an announcement on protecting the environment.

Most are predicting that by 2015, cars and trucks together would have to average nearly 30mpg to be able to reach the 35mpg by 2020 deadline. According to The Detroit News, the interim levels could be set as high as 35.7mpg for passegner cars and 28.6mpg for light trucks by 2015. Today the standard for cars is 27.5mpg and for trucks is 22.5mpg. Sadly, the uncertainly of the new standards has forced a number of carmakers to put development plans for several new models on hold, most notably GM and its RWD sedan and sports car program.