President Barack Obama's EPA now looks likely to reverse Bush and allow California and 13 other states to set their own stricter auto emission and mileage standards. By 2016, the California rules will require automakers to show a 30 percent overall reduction in their vehicles' greenhouse gas emissions. That's not bad considering that at least 40 percent of the 16 million new cars sold each year come from states that want to adopt the California standards (and that the California rules might just become the national default anyway). The transportation sector accounts for 26 percent of U.S. greenhouse gas emissions--the largest single chunk after the catch-all category of "industry."
By 2015, the California rules are at least 3 miles per gallon stricter than federal standards. That translates into a first-year savings of at least 200 million gallons of gas. It doesn't seem like much when you consider that each day the United States uses about 390 million gallons of gas, but the savings will grow each year as more new cars hit the road, until 2020, when the California standards become 7 mpg more stringent than the federal rules and things really get interesting.