However, a number of secondary consequences or consumer reactions may lessen this impact. First is the “rebound effect.” A rebound effect occurs as a result of vehicles getting better fuel economy, so they are driven more. This results in lower than expected net energy savings. Cost savings only can be realized for the average consumer if the total miles driven does not increase as fuel prices fall.

Moreover, emission profiles change. The U.S. Department of Transportation estimates that carbon monoxide increases may result from the more miles traveled by the vehicles with higher fuel economy. Reductions in other pollutants, such as sulfur dioxide, particulate matter and carbon dioxide levels, generally fall as expected.

The 2007 EISA bill also contains a renewable fuel standard (RFSII) use of 36 billion gallons by 2022, with 21 billion gallons coming from nonstarch sources such as cellulose or sugar.

There is quite a debate within the renewable fuels industry whether fuel mileage declines with the greater use of ethanol and next-generation fuels. Initially, fuel economy suffered in gasoline- burning vehicles because ethanol has lower energy content than petroleum-based fuels.

However, new flex-fuel vehicles are optimized for renewable energy use, and several tests have found that the mileage is equivalent to gasoline. Nevertheless, the American car fleet still is dominated by older gasoline models. So, while the EISA attempts to improve CAFE car mileage, greater consumption of renewable energy works against this national goal.