Currently the National Highway Traffic Safety Administration (NHTSA) has responsibility for writing and enforcing fuel economy rules while the Environmental Protection Agency (EPA) oversees emissions regulations. The actual testing of vehicle fuel economy is done by the EPA as part of their emissions test process. Dingell wants to ensure that the precise rules written by the agencies are consistent and don't give conflicting guidance to automakers. Presumably this means that he doesn't want the EPA to establish CO2 limits that are tougher than the the fuel economy rules written by NHTSA.
[Source: Detroit News]