Quote Originally Posted by cstone View Post
Does that make the engine more efficient? More MPG or more HP per gallon of fuel burned?

It seems that the more NOX reduction the more CO2 produced and vice versa. Pick your poison but I would think that burning less fuel is the best route.
No, it doesn't make them more efficient. Sort of apples to oranges, but my Dodge had the DPF/EGR only system, and our Chevy work trucks have the DPF/EGR/DEF system that all new diesels have. The new systems get worse mileage than my Dodge, in factory condition.

The EPA doesn't care about efficiency. They care about exercising their might within their fiefdom.