EPA 2010 will increase fuel mileage for manufacturers using SCR. They are able to increase the efficiency of the engines via tuning lower EGR rates, producing more NOx. However, because they are now treating it out of the cylinder with the SCR, the NOx is not an issue. With the engine tuning and lower EGR rates the engines will also produce lower amounts of soot, hence less DPF regenerations and less fuel wasted. But, all this comes with more hardware, more computers, added weight, and another tank to fill.
Manufacturers such as Navistar...have chosen to not use SCR and rely on credits and "massive" EGR rates to meet the NOx requirements. Expect their fuel economy to drop even further.
I love tree huggers.:icon_geek: