The Environmental Protection Agency says another arm of the Bush administration may be low-balling the economic benefits of increasing fuel economy standards for cars and trucks.

Echoing criticism previously voiced by Democrats and environmentalists, the EPA said in comments filed with the Transportation Department that the department would have been better off using higher estimates for future gasoline prices when it proposed increasing the average fuel economy of all vehicles to 31.6 miles per gallon by 2015.

The proposed fuel economy increase was based in part on estimates that gas would range from $2.04 a gallon to $3.37 a gallon, averaging $2.42 a gallon in 2016.

The national average price for unleaded gasoline was at $3.68 on Tuesday, according to AAA.

Congress last year required the National Highway Traffic Safety Administration -- an agency within DOT -- to set mileage standards at the "maximum feasible" level each year, reaching a minimum of 35 mpg by 2020, a 40 percent increase over current standards.

If the highway administration uses a higher estimate for gas prices, it could make a more cost-effective case for raising the requirements beyond 31.6 mpg by 2015.

Gas price estimated by NHTSA "are more optimistic than I think any reasonable person would be in this era," said Frank O'Donnell, president of Clean Air Watch.