Emission and mileage are not correlated positively. Healthy exhaust comes at a cost. An engine purely optimized for efficieny would be more efficient than those

Emission and mileage are not correlated positively. Healthy exhaust comes at a cost. An engine purely optimized for efficieny would be more efficient than those that comply with emission regulations. But in practice, of course the old ones are worse at efficieny, too.